The superhet in depth: mixers, image frequency and IF stages
The superhet and double superhet, mixer products, choosing the local oscillator, calculating the image frequency, high and low IFs, and IF transformers and gain control.
Key points
Summary
| Item | Rule |
|---|---|
| LO | fRF + fIF or fRF − fIF |
| Image | 2 × fIF from the wanted signal, beyond the LO |
| High IF | Image rejection |
| Low IF | Selectivity |
| Double superhet | High first IF, low second IF |
| IF coupling | Under: narrow · critical: max, flat · over: double hump |
Quick check
A receiver tunes 7.1 MHz with a 455 kHz IF and the LO above. What is the LO frequency?
- 7.555 MHz
- 6.645 MHz
- 7.1 MHz
- 455 kHz
In that receiver, where is the image frequency?
- 6.19 MHz
- 8.01 MHz
- 7.555 MHz
- 7.1 MHz
What is the main advantage of a high first IF?
- Narrower filters
- Lower cost
- Better image rejection
- Less need for AGC
Work it out
Try each one first, then open it to see the working.
RF 14.2 MHz, IF 10.7 MHz, LO above. Find the LO and the image.
- fLO = 14.2 + 10.7 = 24.9 MHz
- Image = fLO + fIF = 24.9 + 10.7
RF 145.0 MHz, IF 10.7 MHz, LO below. Find the LO and the image.
- fLO = 134.3 MHz
- Image = fLO − fIF = 134.3 − 10.7
RF 7.1 MHz, IF 455 kHz, LO above. Find the image.
- 7.1 + 2 × 0.455
On air
- Find your receiver's IFs in its manual and work out one image frequency.
- Tune to a strong broadcast station and listen for its image elsewhere.
- Compare the passband shape of your widest and narrowest filters.
Video transcript
Almost every receiver since the nineteen thirties has been a superhet. In this lesson: its stages, mixers and their products, choosing the local oscillator, the image frequency, why receivers use two I Fs, and I F transformers.
Signals from the antenna pass through the R F stage, which is tuned to reject unwanted frequencies, especially the image. The mixer combines them with the local oscillator, producing sum and difference frequencies. One of these is the intermediate frequency, which passes to the I F amplifier. Because the I F is fixed, its filters can be made very selective. Then the detector recovers the audio, and automatic gain control keeps the level steady. A double superhet repeats the mixing, with a second oscillator and a second I F.
A mixer produces the sum and difference of its inputs, plus smaller products from their harmonics. For a given R F and I F, there are two possible local oscillator frequencies: the R F plus the I F, or the R F minus the I F. Whichever you choose, another frequency on the far side of the oscillator also mixes to the I F. That's the image frequency, twice the I F away from the wanted signal. With fourteen point two megahertz and a ten point seven megahertz I F, an oscillator at twenty four point nine megahertz puts the image at thirty five point six megahertz. The R F stage must reject it before the mixer.
A high I F puts the image far from the wanted signal, so the R F stage can reject it easily, but very narrow filters are harder to make. A low I F makes narrow selectivity easy, but the image is close, and hard to reject. The double superhet gets both: a high first I F for image rejection, then a low second I F for selectivity. Some receivers go further with a triple conversion.
An I F amplifier uses I F transformers: two tuned circuits coupled by their shared magnetic field. The degree of coupling shapes the response. Loose coupling gives a single narrow peak. At critical coupling, the output is greatest and the peak is flat topped. Over coupled circuits give a double humped curve, wider still. The gain of I F stages is varied by the A G C, but reducing gain by changing a stage's bias can make it distort on strong signals. That's avoided by reducing the gain of the earlier stages first, so later stages never see more than they can handle.
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