The Sherwood table

The comparison of transceivers in the Sherwood table is a repeating discussion on our national repeater PI2NOS in the Netherlands. Soon after the word Sherwood tables is mentioned a sudden moment of silence occurs, probably because ham operators immediately start to look in that table whether or not they can spot their gear and compare it to other receivers. The table is intended for measuring the quality of the receiver. There is no unique way as far as I know on how to do this. From a technical point of view Sherwood engineering in Colorado has chosen for the field dynamic range narrow space. The table can be found here, I found the table via hamnieuws because the latest icom-7300 transceiver just appeared in the table. The next thing you hear on PI2NOS is that some icom-7300 already appeared on tweedehands.nl which is equivalent to ebay or the craigslist in other countries.Speculation starts, and it continues for a few days until someone mentions the word: yes, but the Sherwood table said. Well they do, but what does Sherwood not tell you.

Did the table make sense to you? In that case congratulations. Did you base your decision on buying a radio on the Sherwood tables? Well, most of us did not. When I brought my radios I looked at price and performance, I wanted to have a base-station that was affordable, and that performed both on HF and VHF/UHF. A few choices remain and you decide whatever the budget allows. I still think this is the best way to make a decision.

Is sensitivity of the receiver a serious selection criterion? Theoretically it is, in reality it is not. Depending on the IPO setting of my FT-991 the sensitivity varies between -123 db to -143 dbm. The theoretical limit is -174 dBm for 1 Hz bandwidth, so -174 dBm/hz which you can obtain with the equation P = k.B.T where P is represented in Watt, k is the Bolzmann constant 1.38064852 × 10-23 m2 kg s-2 K-1  the temperature in Kelvin is T and the bandwidth is B. Take a room temperature, T=273+20=293K and a bandwidth of 1 Hz, the value in dBm/Hz for P is 10*log10(k*T*B/1e-3) and we arrive at the -174 dBm/Hz. All receivers in the table are at least 30 dB larger than the room temperature noise floor because of the way Sherwood measured the noise floor. The crux is that the measurement is done with a 500 Hz CW filter, the theoretical noise floor is therefore P = 10*log10(k*293*500/1e-3) = -147 dBm. Better than this is not possible unless you do something against the noise of the amplifier for instance by cooling it down. We don’t, but radioastronomers do.

My Yeasu FT-991 has a noise floor of -143 dBm which is when AMP2 is selected rather than IPO or AMP1. IPO is oftentimes good enough for the stronger stations, and AMP1 is what you often use, but AMP2 is usually noisy so that you’re better off attenuating the signal. It depends on the frequency band you are working on, I’m talking here about 20m and 40m. If you are really worried about sensitivity then use a low noise amplifier LNA to counteract the cable losses between the antenna and the receiver. Usually we don’t use a LNA unless you’re working with frequencies for instance in the L-band. GPS receivers have for this reason a LNA directly near the patch antenna.

Next to sensitivity there is in any superhetrodyne receiver the noise inserted by the local oscillator. The LO phase noise is mostly determined by the noise of the (thermo-stabilized) quartz crystal and the noise of the phased locked loop of the circuit that generates the signal going into the mixer. It is measured as dBc/Hz, so dB’s relative to the carrier and then as a spectral density.  The measurement of the phase noise is performed over 10 kHz bandwidth if I understand the description of the Sherwood table correctly. I’m not sure how the measurement is done, probably you need to open the radio and measure the LO output directly.

I don’t think that sensitivity and local oscillator noise are anymore an issue there days, sensitivity is sort of a non issue in urban areas where every device on the block generates some QRM that may be picked up, but sure, if we would go to the field and run the receiver on batteries then sensitivity is the factor that counts.

What counts more for radios in general is the ability to separate signals, and for this most receivers have a seemingly infinite amount of filters that apply at the HF pre-amplifier, and the pass band filters that play a role at the intermediate frequencies. This is also the criterion that Sherwood takes for comparing different receivers. In the end they come up with the dynamic range narrow space in dB. If one would take two nearby transmitters, then how do they affect the  noise floor of the receiver? This is certainly an issue in particular on the crowded 40m band in the evening in Europe. And this is where you can try to compare receivers provided that the roofing filters are comparable.

But this is also my critique on the Sherwood table, they do not look into the possibility of digital signal processing, noise blanking or notch filters which is what you often resort to in order to suppress noise of a neighboring transmitter which may cause some splatter. On the other hand, it is true that one receiver may perform better than another based on the filtering capabilities of the radio, and this mostly determines the standing in the Sherwood tables. At this same time I have my doubts whether filter response is the only thing that counts, mostly for two reasons.

Digital signal processing, noise blackers and contour filters on the FT-991 are able to take away a significant part of the noise to retain that part of the spectrum that you want to concentrate on. I use it quite a lot to isolate weak stations, thus DSP removes the noise and the audio suddenly becomes readable. The Sherwood tables don’t mention the word DSP, maybe the idea is that digital signal processing is not like a filter and that this is not what the classical analog receiver is supposed to do.

A second point is, if you operate a SDR or if take the audio spectrum from a standard SSB receiver and feed it to your PC then more can be done than just analog filtering. Any spectral analysis software can isolate very signals, this is for instance what PSK reception is based on. Also CW is filtered out in this way by software on my PC, a FIR filter is used to recognize weakest CW signal that are just a few 10’s of Hz apart. Analog filters don’t unless you specifically incorporate them in the design of the receiver, my 10 euro DCF77 time receiver that I once ordered from conrad or that you find in your alarm clock is based on this principle. The receiver is only able to listen to a time signal, and the bandwidth is more narrow than the CW filter.

An analogy: in GPS we have a processing gain that is unrelated to the performance of analog filters. The BPSK of the GPS satellite is such that the transmitted signal is spread over 10 MHz so that the PRN signal arrives below the noise floor of -174 dBm/Hz at the GPS antenna. The LNA in the antenna amplifies the PRN signal that is hidden in the noise, but it is code correlation in the receiver that finally reveals the signal. The processing gain by code correlation in any GPS receiver is 43 dB for the C/A code which is a 1MHz BPSK signal, you calculate it as 10 log10(2e6/100) = 43 dB, which is is enough to lift the GPS BPSK signal above the thermal noise floor of the receiver.

Lets return to PSK. It is also true that the quality of the receiver to suppress man made noise (QRM) from neighboring transmitters does affect the PSK. Oftentimes an automatic gain control is more than a nuisance than that it is a blessing, because the AGC in the the FT-991 does reduce the amplification and push a weak PSK station into the noise whereas it is not. In that case it helps that you can turn off or slow down the AGC, my FT-991 fortunately can. What often also works is to apply a CW filter rather than a full 3 kHz audio spectrum from the SSB receiver. However, in CW you can not transmit audio on the FT-991 so that I can only listen to the very weak PSK stations when the QRM is significant.

In the end what counts for reception is your local environment, the antenna, QRM caused by your household and the neighbors, the solar flux index (which we want high), the magnetic field index (which we want to be low), atmospheric noise in general which we want to be low, and maybe your luck in finding that weak QRP station before the pile-up starts. This is what you can not find in the Sherwood tables, it is also not a property of receivers, true, but maybe it was good to remind the buyer of a potentially too expensive radio.

Finally I could not resist to compare my airspy SDR receiver to the FT-991 and the websdr at the University of Twente. If you don’t own a good receiver then pay a visit to a websdr. I found that the airspy noise floor is certainly elevated relative to the FT-991, but also, any SDR can suddenly show signals at frequencies where the FT-991 and the websdr’s say that nothing is going on. Oftentimes this is a SDR problem of setting the gains correctly, the gain at the HF pre-amp, the mixer gain and the IF gain. What also helps is oversampling in the SDR and allowing a high resolutionin the waterfall plots. Just a waterfall plot on the PC is a remarkable asset, it does not need to be a receiver capability because oftentimes we have a PC running next to the transceiver. Just ground your PC to keep the noise floor down.

The comparison of a websdr to your own receiver is something I certainly recommend. Twente’s websdr tends to pick up more the German stations which are visible within its local horizon, I tend to pick up the stations that are further away and depend on signal propagation and the ionosphere to arrive in Rotterdam. Twente ‘s websdr oftentimes does not hear me while stations 1500 km away do hear my transmissions. Therefore I also watch websdr’s that are located furher away from Rotterdam, for instance in Sweden where you can oftentimes hear your own SSB transmission.

Last update : 1-May-2016, added some details on digital signal processing

Image: http://folio.nzz.ch/2014/august/robin-robin

 

 

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