## Estimation of the contribution of the frontend to the total noise figure

In a radio receiver composed of two stages, the total noise factor $$F$$ can be computed using Friis’s formula as$F = F_1 + \frac{F_2 – 1}{G_1},$where $$F_1$$ is the noise factor of the first block, $$G_1$$ is the gain of the first stage and $$F_2$$ is the noise factor of the second stage. If $$G_1$$ is large enough, then the contribution of the second factor is small and the total noise factor of the whole system is essentially the same as the noise factor of the first stage. This is the reason why a low noise amplifier is useful as a frontend, because it has a low noise factor $$F_1$$ and high gain $$G_1$$.

If $$F_2$$ and $$G_1$$ are known (perhaps only approximately), then it is easy to check if the contribution of the frontend to the total noise figure is large enough so that the total noise figure is determined by the noise figure such frontend alone. However, it may happen that one or both of $$F_2$$ and $$G_1$$ are not known. In email communication, Leif Åsbrink mentioned that there is an easy way of checking the contribution of the frontend without knowing these parameters. The method is to switch off the frontend and note the drop in the noise floor. He gave the following estimates: if the noise floor drops by more than 10dB, then the total noise figure is the same as the noise figure of the frontend up to 1dB; if the noise floor drops by more than 17dB, then the total noise figure is the same as the noise figure of the frontend up to 0.1dB. Here I present the maths behind these kind of estimates.

## Building the G0MRF comb generator

A comb generator is essentially an RF oscillator whose output is amplified in a non-linear fashion, so that plenty of harmonics are produced. This is an easy way of producing microwave signals. The G0MRF comb generator was originally designed as a 2.4GHz signal source, to help in the alignment of receivers for the amateur radio satellite S-band. It has a 96.013MHz crystal oscillator, so its 25th harmonic falls at 2400.325MHz, right in the satellite allocation of the 13cm amateur band. Its harmonics are usable up to at least 10GHz, so this can be an useful tool when working with microwave equipment.

In fact, several other harmonics fall on the amateur bands: The 13th harmonic is 1248.169MHz. This is inside the 23cm amateur band, but quite far from the narrow-band segment, which is at 1296MHz. The 24th harmonic is 2304.312MHz. This falls inside the 13cm band. Indeed, 2302MHz is used for EME in some parts of the world. The 36th harmonic is 3456.468MHz, in the 9cm band, but far from the narrow-band segment. The 59th harmonic is 5664.767MHz, in the 6cm band. This is in the satellite uplink segment and quite near the first narrow-band segment, which is at 5668MHz. The 60th harmonic is 5760.780MHz, right in the second narrow-band segment of the 6cm band. The 105th-109th harmonics all fall in the 3cm band. In particular, the 108th harmonic is 10369.404MHz, which is in the narrow-band segment of the 3cm band.

This is a nice kit which is quite easy to build. Most of the components are through-hole, and it can be put together fairly quickly. I built my kit during the Christmas holidays, but I’ve had the PCB lying around until I installed it in a project box yesterday. Here I describe the kit briefly and show the extruded aluminium case I’ve used.

## Band switching with the Hardrock-50 HF amplifier and the FT-817

The Hardrock-50 HF is a very nice kit for a 50W amateur HF amplifier covering the 160m through 6m bands. It supports interfacing with the FT-817. With the proper cable, the amplifier can be connected to the ACC jack on the FT-817 in order to do PTT keying and read the BAND DATA signal from the FT-817 to select the proper band in the amplifier’s low pass filter automatically. The connections required are shown in the Hardrock-50 Tech Site. Yesterday, I prepared the cable to interface my amplifier to my FT-817ND. However, I have found a problem in my amplifier that prevents automatic band switching from working properly. Apparently, this problem has been fixed in the newest units and can be fixed with an easy modification in the units which are affected.

## Testing microphone performance for Codec 2

Codec 2 is the open source and patent-free voice codec used in FreeDV, a digital voice mode used in amateur radio. Since Codec 2 is designed to be used at very low bitrates (the current version of FreeDV uses 1300bps and 700bps), it does an adequate job at encoding voice, but can’t encode well other types of sounds, and thus fails poorly in the presence of noise. Hence, microphones which may be good enough for other applications can give poor results when used for FreeDV (if, for instance, they pick up too much ambient noise or have too much echo). This is a small note about how to test the microphone performance for Codec 2.