Introduction to modern receiver

1. Heterodyne Receivers
As shown above, since an interferer may fall merely one or two channels
away from the desired signal, the filter must provide a very high selectivity
(i.e., a high Q) [1].
For example, a 900-MHz GSM receiver with 200-KHz channel spacing must
tolerate an alternate adjacent channel blocker 20 dB higher than the desired
signal. If we want to design a second-order LC filter to suppress this interferer
by 35 dB, the required Q value is 63200, which is difficult to achieve
practically [1].
Moreover, since a different carrier frequency may be allocated to the user at
different times, such filter would need a variable and precise center frequency.
This property is difficult to implement as well [1].
1

2. Thus, channel selection proves very difficult at high carrier frequencies. We
must devise a method of “translating” the desired channel to a much lower
center frequency so as to permit channel-selection filter with a reasonable Q
[1].
At mixer output, there may be (RF + LO) and (RF - LO) products.
Nevertheless, the desired signal is usually (RF - LO). Therefore, a LPF (Low
Pass Filter) would filter out (RF + LO) [1].
After down-conversion mixing, we call (RF - LO) intermediate frequency (IF).
Because RF frequency is NOT equal to LO frequency, we call this architecture
receiver heterodyne receiver. The term heterodyne derives from hetero
(different) and dyne (to mix) [1].
2

3. In more common heterodyne receivers, the LO frequency is variable so that
all RF channels within the band of interest are translated to a fixed IF so as to
not require a filter with a variable center frequency to select the IF channel of
interest and reject others [1]. As shown below:
Nevertheless, the numerous users in all standards (from police to WLAN
bands) transmitting signals may produce a lot of interferers, which may
aggravate sensitivity. To understand the phenomenon, we use the following
equations:
To be brief, the IF is just the difference between carrier and LO frequency.
Thus, whether the signal frequency is above LO or below, it will be translated
to the same IF frequency [1].
3

4. The figure shown above depicts a more general case, revealing that both two
spectra located symmetrically around LO frequency are down-converted to
the same IF frequency. In other words, (LO - RF) = (Image - LO) = IF, so the
IF desired signal will be interfered with the co-channel IF image. Due to the
symmetry, so the blocker is called image. Heterodyne receivers suffer from
this image issue [1].
As shown above, for down-conversion, the LO frequency can be chosen
above or below channel frequency. These two scenarios are called “high-side
injection” and “low-side injection” respectively.
4

5. Due to the symmetric characteristic around LO frequency, if f1 is desired
signal (i.e., high-side injection), f2 is image; if f2 is desired signal (i.e., low-
side injection), f1 is image. Thus, we can express image frequency as:
Because the image power can be much higher than the desired signal. In
addition, the choice of LO and IF frequency can lead to an image frequency in
a high-interference band. Therefore, the receiver must incorporate a means of
suppressing the image. The most common approach is to precede the mixer
with an “image-reject filter” [1]. The more the IF spacing is, the larger the
image rejection will be.
5

6. According to Friis formula, as shown below, the image filter is behind LNA so
that the gain of LNA lowers the image filter’s contribution to overall noise
figure. [1]
Moreover, the image filter can improve the linearity of the receiver chain. The
cascade IIP3 formula is as shown below [2,3]:
If we regard LNA, image filter, and mixer as stage1, stage2, and stage3
respectively, we can find that the overall IIP3 will improve since G2 is smaller
than 1.
6

7. As described earlier, it is desirable to choose a large value of IF frequency to
maximize image rejection. Nevertheless, recall that the premise in a
heterodyne architecture is to translate the center frequency to a sufficiently
low value so that the channel selection by means of practical filter becomes
feasible. [1]
Thus, the pros and cons of high and low IF frequency respectively is
summarized as below:
Thus, heterodyne receivers suffer from a trade-off between image rejection
and channel selection [1].
7

8. Q. Without concern for interferers, is image filter still necessary?
A. Yes. Even though in the absence of interferers, the thermal noise always
exists in all frequency, including image frequency. Thus, the desired signal,
the thermal noise in the desired channel, and the thermal noise in the image
band are down-converted to the same IF, raising the noise floor in IF band
about 3 dB (-174 dBm/Hz – 174 dBm/Hz = -171 dBm/Hz) [1].
Thus, an image filter can remove the thermal noise in image band, that’s why
image filter is still necessary even though in the absence of interferer in image
band.
8

9. In order to achieve good image rejection and channel selection
simultaneously, the concept of heterodyne can be extended to multiple down-
conversions, as shown below [1]:
This architecture is called dual-IF receiver. As shown above, both image
rejection and channel selection are achieved simultaneously.
9

10. However, in a cascade of gain stages, the noise figure is most critical in the
front end and the linearity in the back end. Recall from the cascade IIP3
formula as shown below:
It’s apparent that the numerator of final stage is largest, thereby aggravating
overall IIP3 most. Besides, from the formula, more gain stages lead to worse
overall IIP3 even though in the presence of passive filters.
In practice, mixers are not so ideal, introducing undesirable effects in the
receiver path. Specifically, mixers in fact multiply the RF input by a square-
wave LO. Take Qualcomm WTR4905 for example, the XO signal between
PMIC and transceiver is typically square-wave. So, we need R-C LPF to
suppress these harmonics [4].
Thus, we need to view mixing as multiplication of the RF input by all
harmonics of the LO. In other words, the RF mixer produces components at
(RF ± mLO1). The IF mixer produces components at (RF ± mLO1 ± nLO2),
where m and n are both integers [1].
10

11. As shown above, this is a dual-IF receiver. For the desired signal, of course,
only (RF – LO1 – LO2) is of interest. So, the frequency of image1 and image2
are 1560 MHz and 380 MHz respectively. However, if the frequency of
interferer is 2800 MHz, with 2nd order harmonics of LO2, the dual down-
conversion process will be (RF – LO1 – 2LO2) = 2800 – 1980 – 800 = 20
MHz, which coincides with the desired signal at the second IF.
If the frequency of interferer is 4380 MHz, with 2nd order harmonics of LO1,
the dual down-conversion process will be (RF – 2LO1 – LO2) = 4380 – 3960 –
400 = 20 MHz, which coincides with the desired signal at the second IF as
well [1].
Thus, with harmonics of LO, even though the interferer whose frequency
doesn’t coincide with image, it will also be down-converted to the identical 2nd
IF frequency. This issue is called “Mixing spurs” [1].
11

12. The dual-IF architecture consists of two down-conversion steps, these
additional IF filters and LO further complicate the design and, more
importantly, the mixing spurs issue is difficult to manage. In other words, dual-
IF receiver is a good concept of rejecting image and channel selection
simultaneously, but not practical. For these reasons, most heterodyne
receivers employ merely two down-conversion steps. That is to say, the down-
conversion process will be (RF -> IF -> BB-> Demodulator) [1].
In addition, as shown below, this traditional heterodyne architecture
(RF -> IF -> Demodulator) has been used for many years [5].
It translates an RF input signal to IF, which is then demodulated to allow the
modulation data to be processed [5].
12

13. Spectrum Analyzer
As shown above, this is a simplified block diagram of spectrum analyzer,
which is a traditional heterodyne receiver architecture [6].
The first part is RF input attenuator. Its purpose is to ensure that the RF signal
enters the mixer at the optimum level to prevent overload, gain compression
and distortion.
The LPF blocks high-frequency interferers from reaching the mixer and
prevents out-of-band interferers from mixing with LO, thereby producing
unwanted responses on the display.
For down-conversion, tuning the spectrum analyzer to the desired frequency
range is a function of the center frequency of the IF filter and the frequency
range of LO.
13

14. As mentioned earlier, in common heterodyne receivers, the LO frequency is
variable and IF frequency is fixed [6].
As shown above, the LO frequency is not high enough, so there is no
response on the display. The LO must be tuned to the frequency of (IF +
Signal) to produce a response on the display.
After down-conversion, the next component is VGA (Variable Gain Amplifier).
When the IF gain is changed, the value of reference level is changed
accordingly to retain the correct indicated value for the display signals. But,
generally, we don’t want the reference level to change when we change the
input attenuator, so the settings of the input attenuator and IF gain are
coupled together. A change in input attenuator will automatically change IF
gain to offset the effect of the change in input attenuation, thereby keeping the
signal at constant position on the display [6].
14

15. After the IF gain amplifier is IF filter. The bandwidth of IF filter determines
frequency resolution, which is the ability two separate two closed tones. As
mentioned earlier, both the IF frequency and IF filter center frequency are
fixed. Nevertheless, the IF filter bandwidth is selectable, thereby selecting one
sufficiently narrow bandwidth to resolve closely spaced signals [6]. The
bandwidth of IF filter is called RBW (Resolution Bandwidth).
As shown above, two signals are separated by 20 kHz. Three traces with
different colors represent three different RBWs: 30 kHz (blue), 10 kHz
(yellow), and 3 kHz (pink). We can observe that smaller RBW leads to better
frequency resolution, and the signal separation is only apparent when the
RBW is less than the frequency difference [7].
15

16. Moreover, smaller RBW reduces the noise energy and as a result, the
average noise level becomes lower, which is beneficial to observe weak
signal [7].
After IF filter is envelope detector, which extracts envelope from the signal.
16

17. The component follows envelope detector is video filter, which is able to
reduce the effect of noise on the displayed signal amplitude with variable
bandwidth, thereby smoothing or averaging the display. As shown below [6]:
The video filter is a LPF and determines the bandwidth of the video signal that
will be digitized to yield amplitude data. The bandwidth of video filter is called
VBW, which affects smoothing effect [7]. As shown below, smaller VBW leads
to better smoothing effect.
17

18. 18

19. Besides, VBW-to-RBW ratio is a parameter affecting smoothing effect as well.
As shown above, the three traces with different colors represent different
VBW-to-RBW ratio respectively: Green (0.01), yellow (0.1), and blue (3). It is
apparent that smaller ratio leads to better smoothing effect [6].
19

20. One of GSM transmitter test items is “TX Noise In RX Band”. As shown below:
Because the limit is stringent, the noise floor on the display is critical. As
mentioned earlier, to narrow RBW can reduce the noise level. Nevertheless,
according to the GSM specification, the measurement must obey the
regulation of RBW and VBW (RBW = VBW = 30 kHz). By the way, as
mentioned earlier, the signal separation is only apparent when the RBW is
less than the frequency difference. GSM channel spacing is 200-kHz, so the
RBW must be less than 200-kHz.
But, we can still set detector type as video averaging, which is best for
observing those signals near noise level [6].
20

21. Now that smaller RBW can lead to better frequency resolution, and smaller
VBW can provide better smoothing effect, why not use the smallest RBW and
VBW for all measurements?
From the formula, we can observe that sweep time depends on span, RBW,
and VBW. It means that the sweep time will be very long with small RBW and
VBW. For example, some noises appear at random time slot, not all the time.
If the sweep time is too long, it’s difficult for the spectrum analyzer to capture
the noise, and there will be no response on the display.
Thus, you have to adjust the RBW and VBW so as to provide a balance
between sweep time and frequency resolution [7].
21

22. Image-Reject Receivers
“Image-Reject” architectures reject image without filtering, thereby avoiding
the trade-off between image rejection and channel selection [1].
One image-reject architecture is Hartley architecture. As shown below:
It makes use of two mixers with their LOs in a quadrature phase relationship;
this separates the IF signal into in-phase(I) and quadrature(Q) components. It
then shifts the Q component by 90 degree before recombing the two paths,
where the desired signal, present in both paths with identical polarities, is
reinforced, while the image, present in both paths with opposite polarities, is
cancelled out [10].
The behavior of Hartley can be expressed analytically. Let’s represent the
received signal as x(t), which can be expressed as :
22

23. For point A:
For point B:
For point C:
Upon addition of point A and point C, we retain the signal and reject the
image.
The principle drawback of the Hartley architecture stems from its sensitivity to
IQ mismatch [1,11]. The perfect image cancellation mentioned above occurs
only if the two LO phases are in exact quadrature and the amplitude in point A
and C are identical. If LO phases don’t accord with the condition, this status is
called “IQ phase mismatch”. If the amplitude in point A and C are not identical,
this status is called “IQ gain mismatch”. If there is IQ mismatch issue, then a
fraction of the image remains [1].
23

24. As shown below, the IRR (Image Rejection Ratio) is defined as:
Amplitude error, ϵ, and phase error,Δθ. It’s apparent that IRR is related to IQ
mismatch. As depicted below [12]:
24

25. The Hartley architecture depicted above is (RF -> IF->Demodulator) form. For
(RF->IF->BB->Demodulator) form, the block diagram is shown below:
With various mismatches arising in the LO and signal paths, the IRR typically
falls below roughly 35 dB. This issue and a number of other drawbacks limit
the utility of the Hartley architecture [1].
25

26. The dual of the Hartley architecture, known as the Weaver image-reject
receiver, achieves the relative phase shift of one path by 90 degree by the use
of a second LO to achieve the same result [1,10].
The behavior can be expressed analytically as well. The final result is:
Thus, we retain the signal and reject the image.
Similarly, IQ mismatch between the two branches can also aggravate IRR in
Weaver architecture, typically falling below 40 dB [1]. Moreover, as mentioned
earlier, dual-IF architecture receivers suffer from mixing spurs in both down-
conversion steps. These drawbacks limit the utility of Weaver architecture.
26

27. Direct-Conversion Receivers
As the name implies, the “direct-conversion” means that an RF signal is
directly down-converted to a BB (baseband) signal without any IF stages, and
therefore it is also referred to as “zero IF” or “homodyne” architecture [12].
Due to the absence of IF stages, direct-conversion architecture is very
suitable for integration as well as multi-band, multi-standard operation. Take
Qualcomm WTR2965 for example, it is just direct-conversion architecture
[13]:
27

28. Nevertheless, direct-conversion architecture brings its own set of issues as
well [10].
28

29. As shown above, due to finite isolation between the RF port and LO port of
down-converter, a certain amount of LO signal leaks to its RF port, RF SAW
filter port, LNA port, and even antenna port. The reflected LO leakage signal
mixes with the original LO signal at down-converter, thereby producing a DC
component (LO – LO = 0 Hz) [12]. This phenomenon is referred to as LO self-
mixing. Take Qualcomm WTR2965 GSM mode receiver for example, LO-to-
RF leakage is almost less than -60 dBm [13].
29

30. Besides, in the full-duplex transceiver, there exists another potential self-
mixing － i.e., TX leakage self-mixing. The first TX leakage path is from the
duplexer through the LNA and RF SAW filter to RF port of the down-converter.
The TX leakage through this path mixes with its feedthrough from the RF port
to the LO port of the down-converter, thereby producing DC component [12].
The second TX leakage path is from PA through substrate, PCB, or power
supply circuitry to LO port of down-converter, thereby producing DC
component in a similar way as the first path. Moreover, the TX leakages of the
first and second paths mix together as well, thereby producing DC
component.
30

31. When a mobile station is operating, it possibly suffers from some strong
outside blocker attack. The blocker may cause self-mixing as well, thereby
producing DC component [12,14].
Utilizing ac coupling In the BB block is an efficient approach to eliminate DC
component. This will be a RC HPF form:
31

32. Nevertheless, a drawback stems from its slow response to transient inputs.
According to the formula:
Because the down-converted BB signal is near DC, with a so low corner
frequency, both the values of resistor and capacitor are extremely large.
According to time constant definition:
The response will be very slow, which is adverse to LNA gain mode switching.
When a mobile station is operating, the LNA input signal level is variable. So,
with weak input signal, the gain should be large enough to lower the overall
noise figure to achieve acceptable sensitivity. Conversely, with strong input
signal, the gain should be low enough to prevent later stages from saturation.
32

33. Thus, take Qualcomm WTR2965 WCDMA mode receiver for example, there
are three gain modes: G0, G1, and G2, for WCDMA operation [13]. If LNA
gain mode switching is too slow, perhaps the gain is low when input signal is
weak, and the gain is high when the input signal is strong. Incorrect LNA gain
mode switching leads to poor sensitivity. Moreover, slow response is harmful
to TDD system because all transients should settle out upon power-up of the
receiver before data reception begins [10]. Therefore, instead of using ac
coupling, there is a built-in DC offset cancellation circuitry in today’s direct-
conversion receivers [1,15].
33

34. As shown below, with DC cancellation, the BER can be lower with the
identical carrier-to-interference ratio [10].
34

35. Second-order distortion is another severe threat to the direct-conversion
receivers [1,12]. As shown below, two strong interferers with narrow frequency
spacing experience 2nd order nonlinearity of LNA, thereby producing IMD2
product near DC [1,16].
Similarly, due to 2nd order nonlinearity of mixer, the IMD2 near DC still occurs
even though LNA is free of nonlinearity [16].
35

36. For FDD system, such as CDMA, WCDMA, and FDD-LTE, a mobile station
receives weakest signal while locating farthest from the base station. And the
mobile’s transmitter is kept close to maximum power to retain communication
quality [16]. As mentioned earlier, there is TX leakage self-mixing issue,
thereby producing DC component. In addition, the LNA gain mode switches to
high gain mode to lower the overall noise figure to achieve acceptable
sensitivity.
36

37. The second order distortion depends on the receiver stages’ linearity. With
LNA’s high gain mode, the linearity is poor. And according to this formula:
poor IIP2 reinforces IMD2 component.
Thus, in today’s direct-conversion receivers, there is not only a built-in DC
component cancellation circuitry, but also an IIP2 calibration circuitry.
= − +
37

38. As shown below, with calibration, IIP2 improves [16].
A vast IIP2 improvement leads to a vast improvement in sensitivity [14].
38

39. Another inherent drawback in direct-conversion receivers is flicker noise,
which is also referred to 1/f noise since it is inversely proportional to the
frequency [12,14].
Flicker noise is contributed by the down-converter, BB amplifiers, and BB
filter, aggravating the down-converted BB signal [12].
The CMOS technology is not suitable for the direct-conversion receivers
requesting high sensitivity, especially for narrow-channel application [12].
39

40. As mentioned earlier, IQ mismatch aggravates IRR for image-reject
architecture receivers such as Hartley and Weaver. This issue limits the utility
of image-reject receivers. For direct-conversion receivers, due to the high
frequency of the LO (i.e., almost coincide with RF frequency), it’s not possible
to implement IQ demodulator digitally [10]. An analog IQ demodulator exhibits
gain and phase mismatch between the two branches [1].
Such imperfections distort the recovered constellation [1,10].
40

41. Thus, analog and digital (DSP based) calibration methods have been
described so as to correct these imperfections [10,17].
41

42. Low-IF Receivers
Low-IF receiver architecture is an offspring of direct-conversion architecture
[1,10]:
The main advantage of Low-IF architecture over the direct-conversion one is
that this architecture has no DC offset problem since the desired signal is off
the DC by the IF. Furthermore, the low-IF architecture is also able to mitigate
the impact on the receiver performance from flick noise near or low frequency
[12].
42

43. As shown above, the Low-IF architecture can avoid these inherent drawbacks
in the direct-conversion receivers. The IF can be as low as half of the desired
signal bandwidth [12]. Take GSM standard for example, the IF can be 100
kHz (half of 200 kHz). With such an IF, on-chip high pass filtering becomes
feasible [1]. Furthermore, because narrow channel standard is more sensitive
to co-channel noise than wideband channel one, low-IF architecture is
particularly attractive for narrow-channel standards, such as GSM [1].
43

44. Thus, MediaTEK (MTK) often make use of Low-IF architecture for 2G
operation.
Nevertheless, as mentioned earlier, low IF means that the image frequency is
very close to desired signal, and this is harmful to image rejection.
Take previous GSM case for example, the IF is merely 100-kHz, the distance
between desired signal and image is only 200-kHz, which coincides with one
channel spacing. Fortunately, according to GSM standard, the receiver can
tolerate the adjacent channel noise which is higher than the desired signal.
44

45. Furthermore, there are several remedies to solve image issue in Low-IF
architecture receiver, one of which is to move the 90 degrees shift in the
Hartley architecture in digital domain [1].
This approach proves a viable choice due to the removal of analog IQ
mismatch. Another approach is to make use of polyphase filter, either passive
or active type [18].
45

46. Reference
[1] RF Microelectronics 2nd edition, Razavi
[2] RF Design - Cascaded Stages
[3] Sensitivity or selectivity - How does eLNA impact the receiver performance
[4] Analysis of GSM ORFS issue
[5] The Differences Between Receiver Types
[6] Spectrum Analysis Basics, Application Note, Keysight
[7] Spectrum Analyzer Basics: Bandwidth, SIGLENT
[8] Optimization of weak signal measurement by spectrum analyzer,
MICRONIX
[9] All-digital PLL and transmitter for mobile phones, IEEE
[10] On the Direct Conversion Receiver -- A Tutorial
[11] Simplified Transceiver Architecture
[12] RF SYSTEM DESIGN OF TRANSCEIVERS FOR WIRELESS
COMMUNICATIONS
[13] WTR2x55/WTR2965 Wafer-level RF Transceiver Device Specification,
Qualcomm
[14] SAW-less Direct Conversion Receiver Consideration
[15] LTC5584 - 30MHz to 1.4GHz IQ Demodulator with IIP2 and DC Offset
Control, Linear Technology
[16] CDMA Zero-IF Receiver Consideration
[17] A single-chip digitally calibrated 5.15-5.825-GHz 0.18-um CMOS
transceiver for 802.11a wireless LAN, IEEE
[18] Low-IF Receiver Planning for The DECT System
46