2.1 Superheterodyne Receiver Architecture and Block Diagrams

2.1 Superheterodyne Receiver Architecture and Block Diagrams

ACMA Exam Focus: Syllabus item 5.16 - identify the stages of a superheterodyne receiver on a block diagram and state the basic function of each stage (antenna input, RF amplifier, mixer, local oscillator, IF amplifier, detector/demodulator, AF amplifier and power supply), and explain what an image frequency is and how it is rejected.

Every transceiver a VK Standard licensee is likely to operate - an HF rig covering 3.5 MHz to 28 MHz, a 6 m all-mode, or a 2 m handheld - is built around the superheterodyne receiver, almost always shortened to superhet. The name joins "super-audible" and "heterodyne": two radio frequencies are beaten together to produce a third frequency that sits above the audio range. Standard Theory questions normally appear as a block diagram with one box left blank, or as a plain-language question asking what a named stage does. Learn the order of the stages and the one-sentence job of each.

Why not just amplify the incoming signal?

The obvious design is the tuned radio frequency (TRF) receiver: several tuned amplifiers in cascade, all ganged to the tuning knob, feeding a detector. It works, but the tuned circuits must track each other across the whole tuning range, gain changes markedly from one end of a band to the other, and the stages tend to break into oscillation because every one of them operates at the same frequency inside a single box. Armstrong's superhet sidesteps all of that by shifting whatever signal you are listening to onto one fixed frequency, the intermediate frequency (IF), and doing the demanding amplification and filtering there.

The stages in order

Trace the path in words: antenna to RF amplifier to mixer to IF amplifier to detector to AF amplifier to loudspeaker, with the local oscillator feeding the mixer from the side and the power supply feeding everything. That sentence answers most block-diagram questions on its own.

Tuning: the local oscillator does the work

The mixer is a non-linear stage, so its output contains the two input frequencies, their sum and their difference. A filter after the mixer keeps only the difference, and that difference is the IF:

• f_IF = | f_LO - f_signal |

Because the IF is fixed by the filter, tuning the receiver means moving the local oscillator so that the difference always lands on the IF. With a 455 kHz IF and high-side injection (LO above the signal), listening on 7.090 MHz in the VK 40 m band requires an LO on 7.090 + 0.455 = 7.545 MHz. Move to 7.150 MHz and the LO must move to 7.605 MHz. Nothing else in the receiver changes.

The image frequency

The mixer works on the difference between the two inputs and does not care which one is higher. That creates the superhet's one unavoidable weakness, the image frequency: a second, unwanted input frequency that produces exactly the same IF output as the wanted signal. It lies on the opposite side of the local oscillator, one IF away, which puts it two IFs away from the wanted signal:

• f_image = f_signal + 2 x f_IF (high-side injection)
• f_image = f_signal - 2 x f_IF (low-side injection)

Worked example 1 (HF). Wanted signal 7.090 MHz, IF 455 kHz, high-side injection. The LO sits on 7.545 MHz. The image is 7.090 + (2 x 0.455) = 8.000 MHz. Check it: | 8.000 - 7.545 | = 0.455 MHz, the same 455 kHz the wanted signal produces. A strong shortwave broadcaster on 8.000 MHz would be heard right on top of the 40 m contact.

Worked example 2 (VHF). A 2 m receiver on 146.500 MHz with a 10.7 MHz IF and high-side injection uses an LO of 157.200 MHz. Its image is 146.500 + 21.400 = 167.900 MHz, far outside the amateur band and easily filtered.

How the image is rejected

The critical point for the exam is where the rejection happens. Once both signals have been converted, they are identical and no IF filter can separate them. Image rejection must therefore be done before the mixer, by the tuned circuits and bandpass filters in the antenna input and RF amplifier stages. Three practical measures follow:

1. Sharper front-end selectivity - band-specific bandpass filters switched in for each amateur band rather than one broad filter.
2. A higher IF - the image separation is twice the IF, so moving from 455 kHz (910 kHz separation) to 10.7 MHz (21.4 MHz separation) makes the front-end filter's job dramatically easier, especially on 10 m and above.
3. Dual conversion - a high first IF such as 45 MHz or 70 MHz for image rejection, followed by a second mixer down to a low second IF such as 455 kHz where narrow filtering is practical.

Modern up-conversion HF receivers take the same idea further: they convert everything from 0.5-30 MHz up to a first IF above 30 MHz, so the image for every amateur band falls outside the tuning range altogether and a simple low-pass filter removes it.