Usually, the multimeter will give you a reading in only one direction: the forward biased direction. Some expensive meters can do reverse biased voltages, i.e. zener diodes, but most multimeters will only do one direction. So, you'd have to connect the positive lead to the anode and the negative lead to the cathode. Then the meter will give you the forward voltage drop of the diode.
If your meter is giving you '0000' both directions, it could mean one of two things:
-- The diode is definitely busted.
-- The meter is not functioning correctly.
Is it possible for you to test the diode function on your multimeter using a known, good diode? That way you can verify at least the meter is working as expected. You don't have to unsolder or take the diode out of the good test board to do this.
Question 1: Assuming that the oscillator (perhaps programmable) is
given, what kind of hardware would you suggest for (2) and (3) ?
For 2: All-pass (phase-shifting) filter, a.k.a. Hilbert transformer
For 3: Balanced modulator
Question 2: Does this setup have significant shortcomings (beside cost
of components?)
Yes. While direct conversion to complex baseband is regularly done in the digital domain, it can be quite finicky to get the same concept working well (and reliably) in the analog domain. It's nearly always easier to do the detection at an intermediate RF frequency.
Question 3: is this observation [about square-wave local oscillators] correct?
Yes, except that you want a bandbass filter that's centered on the carrier frequency, not a lowpass filter. Sometimes this filter has a bandwidth of approximately 2B, in which case it needs to be tuned along with the local oscillator. This sort of setup is called a preselector, and is commonly used in superheterodyne receivers, such as those used for the AM and FM broadcast bands.
But sometimes, a fixed bandpass filter that covers the entire band of interest is used instead. This works as long as none of the unwanted "image" bands ever falls into the band of interest. This is more common in 2-way VHF communications systems, such as those used for air traffic control and public service (police, fire).
Best Answer
Yes.
There are two options for non-reciprocal devices, unfortunately neither is 'simple like a diode', they are an isolator, and a buffer amplifier. Schottky or PIN diodes will not do it, a switch made from them will conduct RF equally well in both directions when on. Attenuators and filters also behave in a reciprocal manner.
The first is a microwave isolator, though these are usually built with a circulator, with its third port terminated. These work over a narrow range of frequencies, in waveguide only, so you would need adapters to get to and from coaxial cable. The magnet can be big and bulky. They are relatively rare and expensive beasts.
The second is an RF buffer amplifier. These have the drawback of requiring power, but they can be very small, and are broadband, covering a wide range of frequencies. Being an active amplifier, they have a limited maximum power handling, add noise and generate distortion. In a pure reverse signal isolation mode, they tend to be used with an additional attenuator, that cancels their gain, and improves reverse isolation. They are available in a huge range from many manufacturers, most will just drop into a 50ohm system, and are relatively inexpensive.
In the very specific case of protecting a receiver from an adjacent transmitter, a limiter, usually made with PIN diodes, can offer a signal level dependent attenuation function. Note that is still not 'direction dependent' per se. For instance, a limiter might be designed to allow no more than +15dBm through it. A +40dBm transmitter will therefore suffer a nominal 25dB attenuation, whereas the receiver signal will not be attenuated.