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I was designing the circuit of a superhet receiver, block by block. Needless to say, my concern is how would I separate the various blocks. In other words, how would I interconnect the blocks ( not just this circuit, but any circuit) so that one block does not affect the other. For my purposes, how would I separate / interconnect :

1) Local oscillator ( Hartley or Colpitt) from a Frequency Mixer ( Ring diode type)

2) The Mixer from active bandpass filter

3) The bandpass filter from the oscillator ( Hartley or Colpitt )

4) Amplification stage from the Mixer

such that each circuit block does its own function without affecting / getting affected ( or getting minimally affected ).

Is there a uniform strategy to adopt ( or a strategy which works in most of the cases ) or does it depend upon the circuit elements involved ?

Will just adding a very high resistance / buffer amplifier between the stages work, or is there more to it ?

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There is no one single strategy, and I don't think you can expect to get the design absolutely right without some trial and error.

Some blocks may need buffering from others. It is common to minimise external influence on the LO, and that often means a buffer between it and the mixer. An emitter follower (or source follower with a FET) running at fairly high current (10-20ma rather than 1-2ma) for good linearity may be called for.

Other times you may simply need to impedance-match between stages, using the "maximum power transfer theorem" (loosely stated, make source and load impedances equal) to minimise added noise by using as much of the source power as possible. This would apply between antenna and input filters, between these and RF amplifier, between mixer and IF amplifier etc, i.e. before the gain stages where signal levels are smallest.

For this reason a lot of RF circuits standardise on specific impedances (50 ohms is commonest, but also 75 ohms, 300 ohms or 200 ohms).

Sometimes you need to match circuits with different impedances. Then you can:

  • tolerate an impedance mismatch. A low source impedance into a high load impedance is safe, but can (a) change the frequency response of a filter (either before or after the interface) or (b) worsen SNR by wasting signal power
  • Add an impedance matching network (can be as simple as a termination resistor). This fixes (a) but not (b) above
  • Match impedances with a transformer. Going from 75 to 300 ohms for example, use a 1:2 step-up transformer ( = 1:4 impedance ratio as the secondary has twice the voltage, half the current). This fixes both issues; if the next stage was the dominant noise source, feeding it twice the input voltage can improve SNR by 6dB!

You will see all these techniques employed.

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Designing stuff by blocks is all very well but at the end of the day it is one circuit and it should all work together. Adding buffers between blocks just to make the "block diagram" meaningful is pointless in the main. Live with the less-than-perfect loading of the mixer on the local oscillator and if necessary alter the L.O. design to give a slightly larger output if that is critical (which it shouldn't be).

A block on its own is pointless - making them work seamlessly with each other (without adding serious amounts of interface glue) is the name of the game.

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  • \$\begingroup\$ I do not understand what you mean to say. My problem is I don't know how to interconnect them. I just can't simply connect them like that. They will affect each others performance. So how do I go about it ? \$\endgroup\$ Jul 10 '14 at 9:43
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    \$\begingroup\$ Without an example circuit I cannot advise you - every example will be different and I'm not going to try and generalize an answer because it will be too long. \$\endgroup\$
    – Andy aka
    Jul 10 '14 at 10:27
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    \$\begingroup\$ Trying to maintain some separation is still a good idea. Too much interaction can result in very unexpected results when a component is changed, making design and debugging much harder. Components are cheap. I'd use a buffer if it saves me some hair-pulling. \$\endgroup\$ Jul 10 '14 at 23:36

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