[TenTec] General Coverage versus ham bands only

[Ken Brown]

One of the big considerations in receiver design is image rejection. 
When the superheterodyne technique was first employed, the received 
signal was down converted to a lower frequency at which hi-Q, stable LC 
tuned circuits, with fairly critical adustments, could be built and 
aligned for that fixed frequency. Since this intermediate fixed tuned 
amplification stage was at a frequency in between the received RF 
frequency and the detected audio output frequency, it was called the 
Intermediate Frequency, or IF. Better selectivity AND frequency agility 
could be realized by having fixed tuned filters in the IF, so that they 
are set when the receiver is built and aligned, and not adjusted during 
operation. Only the local oscillator and perhaps some fairly broad 
bandpass filtering in the mixer or first RF stages was adjusted during 
operation of the receiver. This was usually done using multiple ganged 
variable capacitors. Typically the intermediate frequency was in the 
range of 50 kHz to 500 kHz. Probably the most common IF frequency ever 
used is 455 kHz.

This scheme works quite well for medium wave AM broadcast receivers. To 
down convert a 1 MHz signal to 455 kHz a local oscillator frequency of 
1.455 MHz can be used . (Ever notice how a lot of multiple gang variable 
capacitors have one section with much smaller plates? That is for the 
local oscillator operating at a frequency above the receive frequency) 
With an LO or 1.455 MHz  both 1.000 MHz and 1.910 MHz could be down 
converted to an IF of 455 kHz. One is the desired signal and one is the 
"image" response. The image is 2X the IF frequency away from the desired 
signal. In this scenario the image is almost a full octave away, so the 
tuned mixer or RF stage can easily reduce the image response to an 
acceptable level. When the same low IF frequency is employed in a 
shortwave receiver, the image is still 910 kHz away from the desired 
frequency, and 910 kHz is a much smaller proportion of, say, 7 MHz. 
Nowhere near an octave away. The tuned RF and mixer stages would have to 
be much higher Q to acheive the same image rejection ratio. Much more 
difficult to have the tuned circuits using multiple ganged capacitors 
track each other precisely enough when the Q gets higher. (Ever notice 
how some multiple ganged variable capacitors have slots in the outer 
plates of each section, that can be bent to make fine adjustments of 
that section for a particular shaft position?) So cheap shortwave 
receivers using to old MWBC standard IF usually had very poor image 
rejection.

One solution is to use a higher IF. Since the image is 2X the IF away 
from the desired signal, it can be rejected more easily when the IF is 
higher. If you use a high enough IF, and get the image far enough away 
from the desired receive frequency, you can use fixed tuned bandpass 
filters in the RF and mixer stages, or broadband transformers, and throw 
out those multiganged vairable capacitors. (Really they go in the junk 
box and later become loading capacitors for 500 watt amplifiers) With 
much higher IF the construction of sharp tuned IF filters becomes 
different. Using LC circuits it is more difficult to get the same 
selectivity when the IF is several MHz instead of a few hundred kHz. So, 
enter the dual conversion superheterodyne. The received signal is first 
converted to a fairly high IF, which helps with the image rejection, and 
then down converted to a lower IF which helps with the selectivity. This 
technique is not without it's pitfalls though. Now there are two local 
oscillators (not including the BFO) and other possible undesired mixes, 
or images. With the proper choice of the IF frequencies the image 
response problem can be managed without too much difficultly as long as 
you don't try to cover the whole HF spectrum. If you make it general 
coverage there are going to be areas where the image rejection is poor, 
or where one or more of the local oscillators or their harmonics 
interfere with the received signal.

You can also use a high IF (such as 9 MHz) without going the double 
conversion route, by using crystal filters instead of LC filters in the 
IF. This will reduce some of the problems with multiple images. Still 
there will be a frequency range where desired receive frequency is very 
close to the image. I would guess that if 30 meters was one of our ham 
bands in the 60s, 9 MHz would not have become such a common first IF.

If you want general coverage you can get around a lot of these problems 
by first upconverting to a really high IF, such as 45 MHz or 70 MHz. Now 
the image is so far away, for whatever frequency you are tuned to in the 
HF range, that images can easily be rejected. The LO has to operate at a 
much higher frequency, and could probably not be made stable enough 
using a mechanically adjusted variable capacitor or permeability tuned 
oscillator. Only a crystal controlled oscillator or a phase locked loop 
oscillator is likely to work with this upconversion scheme. And since 
the conversion is to a higher frequency thats is not intermediate 
between the received frequency and audio, it isn't really and 
intermediate frequency anymore. Still, the habit has been well 
established, so we still call it an IF. At least it is still and 
intermediate stage, so what the heck, call it an intermediate frequency 
anyway. Upconversion solves some problems and creates others. More 
difficult to make narrow filters, noise in PLLs, etc, etc.

DE N6KB