Analog signal formatslinrad accepts digital data in two formats. Real signals and complex signals.
A real signal is just a voltage that changes with time like the voltage across the loudspeaker in a normal SSB receiver.
To produce a real signal one usually uses a filter to select a RF passband from F0 to F1. Typically 10.697 to 10.700MHz. A local oscillator at 10.6968 will convert the passband to the frequency range 0.2 to 3.2kHz. To avoid a false response due to the mirror image (the other sideband) the filter has to attenuate 10.696 and lower frequencies. It is possible but less common to supress the mirror image by phasing methods.
Complex signals are more efficient. Two mixers in quadrature are used to produce two signals conventionally named i (in phase) and Q (quadrature) The two mixers differ in that the phase difference between RF and LO differs by 90degrees between them. One feeds the two mixers with the same signals but one phase shifts the LO or the RF signal by 90 degrees.
The quadrature mixer may operate directly at the signal frequency but it may also operate at some lower frequency where it is easier to make good mixers.
Linrad can handle one or two antennas so 2 or 4 A/D channels are required.
Hardware examples for real signalsFor a single antenna a normal SSB receiver will give something like 3kHz bandwidth using only one A/D channel. linrad with a normal ssb receiver gives some information about what to expect when using this easily available hardware. The AFC and the coherent detect functions may be used in combination with narrow filters to make very weak signals easier to decode. This link gives some information: AFC, locking to and following a signal Some more is here:Normal AFC parameters
Some years ago, sound boards would supply 44.1kHz sampling frequency from two channels maximum. To get the performance required at that time it was necessary to use quite sophisticated X-tal filters to fully utilize the two signals from a cross yagi on EME.
Using a large number of surplus X-tals this high performance X-tal filter allows 2x20kHz bandwidth with an old SB16 board sampling at 44.1kHz.
To convert the filtered signal to audio a really good mixer is required. A good idea is to use CMOS switches like the one of which a pair is used in I/Q mixer for direct conversion radio
Hardware examples for complex signalsVery low cost radio Just using two cheap standard mixers and an X-tal oscillator one can get a surprisingly good radio receiver. No anti alias filters are needed, the ones built into the sound board are sufficient if dynamic range requirements are modest. Without any amplifier at all the noise figure is far from acceptable, gain in the order of 60dB has to be added. Part of the gain is preferrably added at audio frequency.
Optimised direct conversion receiver for 144MHz Although it is extremely easy to make a reasonably good radio with just two standard schottky mixers it is not easy to make a direct conversion receiver that preserves the dynamic range of a Delta44 audio board. The problem is that the local oscillator has sideband noise, and in case very good linearity is desired one needs more LO power for a higher level mixer. Increasing both the LO carrier and the LO sidebands means that the audio noise from mixing the LO with its own sidebands grows quadratically. As it turns out, best performance is likely to be obtained with the cheapest low level mixers such as SBL-1 or TUF-1.
The easiest way to make a very good analog front end for linrad is to make the final conversion from RF to audio at a fixed and low RF frequency.
Mixers built around CMOS switches like this one I/Q mixer for direct conversion radio give very good performance. The local oscillator is very critical because in the conversion to audio any phase noise from the LO will mix with the LO itself and produce noise that will degrade the dynamic range. A good X-tal oscillator followed by s synchronuos counter that divides the frequency by 4 will give an excellent control signal for CMOS switches.
The hardware I am using myself has a frequency synthesiser for the LO and the I/Q mixer for direct conversion radio is a direct conversion radio operating from 6.5 to 15MHz. This radio is limited by the noise sidebands of the LO and therefore it does not require a very low noise audio amplifier. I am using an anti alias filter to improve the useful bandwidth to 94kHz for my Delta44 audio board system. This is adequate for the 6.5 to 15MHz local oscillator but I would not recomend a duplication - it is better to have a fixed LO and use a better audio amplifier like the one in Optimised direct conversion receiver for 144MHz to make an optimised fixed frequency direct conversion radio.
The AF feedback four phase CMOS switch mixer provides extreme dynamic range. This is a new mixer which is part of the 2.5MHz direct conversion radio which is designed to be used as a converter in front of a modified Delta 44 for a high performance receiver covering 2.4535 to 2.5465MHz.