# Difference between tuning and matching for transmitter/receiver circuits

I would like to find a coherent answer to following question:

What is precisely the core difference between tuning and matching in HF electronics, especially in design of transmitter/receiver circuits?

• – jonk
Nov 10, 2021 at 23:18
• What is matching? Do you mean impedance matching?
– jay
Nov 11, 2021 at 4:42

'Tuning' is the process of adjusting the value of a variable inductor or a variable capacitor in an LC circuit (also known as resonant, tank or tuned circuit), in order to vary its resonant frequency. It's the means to select the operating frequency of a receiver or a transmitter.

A tuned circuit enables an RF circuit to be selective about the frequency being passed. In the case of oscillators, LC components set the operating frequency of the circuit.

'Matching' is the process of adjusting the value of a variable inductor and a capacitor in an LC network, connected before the antenna, to make the antenna impedance appear identical to that of the receiver or transmitter. It's the means to maximize the received signal strength or the RF power transferred to the antenna or to minimize the transmitted signal being reflected by the antenna.

Impedance matching is an essential part of circuit design. It enables maximum transfer of signal power between stages.

An 'antenna tuner' is a misnomer as it does not 'tune' the antenna but only 'matches' its impedance to that of the receiver or transmitter.

• Possibly I'm missing a point but when you say that for 'matching' we want 'the antenna impedance appear identical to that of the receiver or transmitter' do you possibly mean not 'identical' but 'complex conjugate' in the sense of equation here: en.wikipedia.org/wiki/Impedance_matching ? Nov 13, 2021 at 2:51
• Hi katalaveino, Load impedance identical to source impedance ensures minimum reflection. Complex conjugate match ensures maxim power transfer. Nov 13, 2021 at 8:36
• I'm confused. Although I agree with you that seeminly the mathematical conditions for maxim power transfer and minimal reflection difer (equality= complex conj), but I not see how physically it can be possible. I thought that the only way for load not to absorb whole tranfered power provided by source, is to reflect some it back, ie that lost power=carried by reflected waves. But here seemingly that's NOT the same and my intution is joking on me. Do you see how one should think about the difference between these two seemingly different methods phenomena one encountern doing impedance matching? Nov 14, 2021 at 2:23
• Hi katalaveino, Here's a good reference that would be of help in understanding the difference between the two. ece.rutgers.edu/~orfanidi/ewa/ch13.pdf Nov 14, 2021 at 5:49

"Tuning" is an improvement of filter or amplifier overall performance in response vs frequency or maximal power output.

Matching Impedance is fairly straight forward for maximum power transfer.

"Matching a filter" is one of the simplest concepts to optimize the spectral Signal to Noise Ratio, SNR, by matching the filter response to the signal spectrum and also matching impedance.

Realization of this response is secondary to defining the specs for the filter.

I would like to find a coherent answer to the following question: the ... difference between tuning and matching

Well, wouldn't we all. It ain't happening though.

Tuning can have any of (at least) three meanings:

• Adjusting the operating frequency to a desired value - "tuning a station"
• Adjusting a circuit for best operation - "tuning an IF strip". This is similar usage to tuning a car engine for racing.
• A straightforward synonym for matching, in the sense described by Asdf - "an antenna tuning unit".

Matching can mean:

• Adjusting the impedance of adjacent stages, to maximise transfer of signal in the desired direction and minimise reflection of signal in the opposite direction (as discussed by Asdf and vuu2nan),
• Matching filter characteristics to signal characteristics (as discussed by Tony Stewart). This might mean choosing a filter bandwidth to suit the signal, e.g. a broadcast receiver might use a 9 or 10 kHz filter for AM signals and a 200 kHz filter for FM transmissions. More sophisticated usage might match the impulse response of the signal and filter e.g. a gaussian or raised-cosine filter. This is a less common usage and is something you choose at the design stage.

So you can pick any mixture of these definitions. Unfortunately you just have to work out the intended meaning from context. It gets easier with experience.

Tuning usually means online change of parameters, like the working frequency.

Matching means matching the input impedance. For passive radio frequency (RF) circuits it is critical for reducing the reflection from the junction. Classic circuit theory tells that at high frequencies it may be achieved by perfectly matching the output impedance (of the generator aka source) with the input impedance of the connected circuit (known as load) at specific frequency. Wideband matching means that the impedances are equal at every frequency in the pass-band. Since the impedance of most transmission lines is more or less constant in a wide band, this means that the input impedance of the load should also be constant. Simplest way is designing the load with constant characteristic impedance (the impedance of the equivalent transmission line) and matching it with the impedance of the source transmission line; the next cascaded load should be matched to the same source impedance for keeping the input impedance of the previous load unchanged. This offers the widest pass band of the circuit. Btw, since antennas are always loaded on the open space impedance of 377 Ohms, it influences the antenna input impedance strongly; so mathching antennas in a wide band is a tricky thing.

Coming back to tuning: rejection usually means reflection from unmatched impedance. Therefore most tuning circuits simply change the characteristic impedance at the pass-band edge (cut-off frequency), making it unmatched there.

Matching of transistor amplifiers is a different thing, since these are strongly nonlinear devices with feedbacks. Depending on what you want to get, most output power, or the lowest noise level, the input impedance of the load may be very different from the amplifier output impedance. It is also very important that the matching fulfills the amplifier stability conditions, since the load of the amplifier is a part of its feedback, and incorrect loads may increase the noise floor, turn the amp. into an oscillator, or even burn it out.

There exists as special class of integral amplifiers called "block" amplifiers, which are noise-, or power-matched (usually in-between) and unconditionally stable inside, with impedance transformers attached inside the package to standard 50 Ohm transmission line impedance.