Overview. NorCal 40A. Direct Conversion vs. Superhet Receivers
The overall objective of this course is to learn and understand
practical aspects of analog wireless communication electronics.
We will accomplish this with a very thorough analysis of the
NorCal 40A transceiver (= transmitter + receiver). This radio
was designed by Wayne Burdick and the kit is produced by Bob
Dyer at Wilderness Radio.
The NorCal 40A is a QRP (= low power) and CW (= continuous
wave) rig. It operates in the “40-m band,” which designates the
wavelength of the carrier. With? = c / f , then f ? 7 MHz.
This frequency is within the HF (= high frequency) band, which
extends from 3 to 30 MHz. In this band, worldwide
communications is possible since the Earth’s ionosphere acts to
reflect the signal back towards the ground .
A block diagram of the NorCal 40A is shown in Fig. 1.13. The
transmitter is on the left half, the receiver on the right. Notice
the different frequencies at various stages in the circuit .
This block diagram is constructed on a system level. Each
shaped section in the diagram serves a specialized purpose .
There are five types of system blocks in
We will briefly discuss the first four of these.
(1) Amplifiers. These are used both in the transmitting and
receiving stages of the transceiver.
Take, for example, the Driver Amplifier:
The amplifier has amplified the transmitted signal by
This amplifier is followed by the Power Amplifier [G =
10log10( 2/ 0.020 )= 20 dB] to give an output power of 2 W into a “well-matched” antenna (2 W is considered QRP).
This type of gain is called an operational power gain. Later in
the course we will almost exclusively use another type of gain
called maximum available power gain.
(2) Filters. These devices are a common topic in early EE
courses. Filters play an extremely important role in analog
communication electronics. In the NorCal 40A, there are three bandpass and one low pass filter. One of the bandpass filters (the IF Filter) is constructed from four quartz crystals and has a very, very large Q for discrete component filters (Qloaded ? 12,000). Recall that
Where
Filters are typically characterized by two factors:
(i) Loss L in the “pass band.” Take a low pass filter for
example:
So, in the pass band
We see that loss L is the inverse of gain
(ii) Rejection factor R in the “stop band.”
where P = power at some frequency f in the stop band
(3) Oscillators. These provide nearly sinusoidal signals at a
single frequency. There are three oscillators in the NorCal 40A:
(i) Transmit Oscillator at 4.9 MHz,
(ii) Variable Frequency Oscillator (VFO) near 2.1 MHz,
(iii) Beat Frequency Oscillator near 4.9 MHz.
(4) Mixers. These are circuits that shift a signal’s frequency
either up or down. This shifting is accomplished by “combining”
the signal with another.
This “combining” operation is signal multiplication and is
usually accomplished either with nonlinear circuits or with timevarying circuits. (The NorCal 40A uses the latter.)
As an example of mixing, let’s take the product of
V1(t)=cos (2p f1t)
And
V2(t)=cos (2pf2t)
As
V(t)=V1(t).V2(t).
Then .
Where
F+=Sum frequency = F 1+F2
F-=Difference frequency =| F 1+F2|
Consequently, through multiplication we have produced an
output signal containing two frequency components:
On a spectrum analyzer: Direct Conversion Receivers
As an application of mixers, consider the “direct conversion”
receiver shown in
The frequencies at different points in the receiver can be drawn
graphically as:
The audio amplifier would amplify the low frequency f- signal
while filtering out the sum frequency f+. In the NorCal 40A, the
audio frequency ? 600 Hz and the RF ? 7 MHz. Therefore:
LO ? 7 MHz and Sum ? 14 MHz.
This simple receiver has one major problem, which is the
“image.” The audio signal is the difference signal for RF and LO
mixer inputs. If a signal is also being received at the “image”
frequency shown above (at the same time as the desired RF
signal), then a second audio tone will be produced at the output
as the difference between the image and the LO frequencies.
This is BAD since you will hear two “stations” simultaneously
and there would be no way to separate them.
One way to circumvent this problem is to place a filter before
the mixer to remove the image, as shown in Fig. 1.10. However,
one would need a very high-Q bandpass filter, most likely
requiring quartz crystals. But then we couldn’t change
frequencies to tune in other stations because it’s difficult to
make such filters with a variable center frequency. Superheterodyne Receivers
This problem with the direct conversion receiver can be
overcome using superheterodyne receivers. This circuit was
invented by Howard Armstrong in the early 1920’s. The
superhet receiver is “perhaps the most important invention in the
history of communications,” as stated in the text. Coincidentally,
Mr. Armstrong also invented frequency modulation (FM).
A block diagram of the superhet receiver is shown in Fig
Consider the various frequencies present in the circuit beginning
at the antenna:
• Stage 1
The RF Filter easily filters out the “VFO image.” The IF signal
is then fed to the IF Filter and the Product Detector
• Stage 2
The IF Filter needs to have a very large Q. Its job is to filter out
the sum frequency signal (f+ = 9.1 MHz) and the BFO image (if
there is one at ? 4.9 MHz).
Notice we can tune this receiver by varying the VFO frequency;
however, the intermediate frequency is ALWAYS equal to the
IF. Consequently, we can construct a very high Q filter centered
at the IF that doesn’t need to be tuned. Brilliant! The loaded Q of the IF Filter in the NorCal40A is approximately 12,000. That’s large!
A block diagram of the superhet receiver is shown in Fig
