AM radio kit -- good explanation, except for the mixing part - Page 1
Forum section: electronics / rf-microwave

author: petert
author_rank: Regular Contributor
timestamp: August 26, 2019, 09:59:41 am
content: Hello,I have been experimenting with radios for a while now, starting with a crystal radio, experimenting with filters and oscillators. But I want to take the next step and understand a heterodyne AM radio, and found an interesting kit:MODEL AM/FM-108CK SUPERHET RADIO for example here:https://www.amazon.com/Elenco-Radio-Kit-Combines-Transistors/dp/B008515U1UThefull instruction manualis available online.While I like the instructions, especially regarding tuning with a scope and signal generator, I find them a bit lacking regarding the mixing part. I cannot see how it is achieved and what type of mixer is used to create the intermediate frequency.Around page 31 a little about this process is explained, but it merely states that 4 types of frequencies are available at the transistor Q7, but not why.I was expecting that it would use some relatively simple circuit like a ring modulator to do the mixing, but it is not obvious to me how this is achieved in this circuit.If anybody knows of a kit where these stages of mixing and filtering are a little more explicit and explained in more detail I would be grateful.I am especially looking for a kit, to avoid getting wrong or unmatched components (e.g. matching diodes for a ring modulator, or being sure the components work for the frequencies in question -- here AM radio range), and thereby reducing the possible points of failure.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: August 26, 2019, 10:51:54 am
content: A ring modulator is used in high performance equipment.Cheap gear rarely uses them. This is because:(a) They require a high level of local oscillator drive (you'd need an extra transistor to amplify)(b) They have a conversion loss (you'd need extra amplifier stages)(c) They use RF transformers which are more expensive to make than other componentsI don't know about that kit but cheap broadcast radios typically use an autodyne mixer where the one transistor is both the local oscillator and mixer.That's hard to understand but saves on parts which is often the No 1 design aim of consumer gear.A small signal transistor would be typical in this role.

author: ledtester
author_rank: Super Contributor
timestamp: August 26, 2019, 11:48:38 am
content: I'll try my hand at explaining this...I believe the mixer here is what's commonly called a "self oscillating single bjt mixer" whose basic idea is this:The transistor is biased to act as an amplifier - the signal at the base of the transistor will appear amplified at the collector. In normal applications the collector load is just a resistor, and this results in simple amplification of the base signal. However, if the collector load is a tank circuit (such as one of those metal can filters), the output will also be filtered around a band corresponding to the resonant frequency of the tank.If another signal is injected into the emitter of the transistor it will vary the gain of the amplifier accordingly to the level of the signal. The result is that the base signal gets modulated by the emitter signal which gives our (product) mixing.You can see this going on in this schematic:The first transistor is the mixer. The RF signal is presented at the base of the transistor, the LO is injected into the emitter and the filter is T3.Here's a discussion and simulation of how this kind of mixer works:https://hackaday.io/project/19254-the-bipolar-junction-transistor-rf-mixerThere is a full schematic on the last page of the kit's manual. You can see signals being injected into the base and emitter of Q7 through C29 and C30/C31. R31/R32 bias the base of Q7 and R33 is the emitter resistor.

author: ledtester
author_rank: Super Contributor
timestamp: August 26, 2019, 12:12:29 pm
content: As to why you see four frequencies at the collector of Q7... here's my explanation:- RF+LO and RF-LO appear due to the product mixing of RF and LO. This is just a trigonometric identity - the product of two sine waves may be expressed as the sum of two sine waves whose frequencies are the sum and differences of the original frequencies:https://en.wikipedia.org/wiki/List_of_trigonometric_identities#Product-to-sum_and_sum-to-product_identities- the signal at the base of Q7 can leak into the collector since the base-collector junction is just like a diode- the signal at the emitter of Q7 is coupled through L5 to the collector

author: radiolistener
author_rank: Super Contributor
timestamp: August 26, 2019, 05:29:38 pm
content: Quote from: petert on August 26, 2019, 09:59:41 amMixing of two signals arithmetically meansmultiplying of one signal with another. It happens on non-linear components such as diode due to it's non-linearity. Multiplication of two real signals with frequencies f1 and f2 produces new frequency products: f1+f2 and f1-f2 on the output (seetrigonometric equationsmentioned above to understand why it happens).This is used to shift some frequency on some offset. For exampel if you want to receive 7000 kHz, you can use LO=7500 kHz, so on the output of mixer you will get:output = 7500 + 7000 = 14500 kHzandoutput = 7500 - 7000 = 500 kHzIt means that on the output of mixer you will get original 7000 kHz signal shifted to 500 kHz and to 14500 kHz simultaneously.Now you can apply bandpass filter with center frequency 500 kHz to cut off all except 500 kHz. With almost the same result you can apply bandpass filter with center frequency 14500 kHz.500 kHz here is your intermediate frequency.Since there is just only your 7000 kHz signal moved to 500 kHz frequency on the output (all other frequencies were cut-off with BPF), now you can just apply AM detector to get HF envelope and you will receive 7000 kHz station in such way.As a side effect, such receiver will be very sensitive to intermediate frequency. So, you're needs to cut off signal with frequency equals to IF from the input signal before put it on the mixer.