Codan 7004 - WSPR 40m
This article describes a project to monitor the 40m WSPR band.
The objective is a dedicated stable WSPR monitoring station limited by ambient noise rather than receiver noise.
Minimum receiver noise figure.
For the receiver internal noise to have insignificant effect on the weakest signals, it must be much lower than the external or ambient noise, say at least 10dB less.
Fig 1 from the article Ambient noise survey, Bowral NSW on 40m, 13/08/2010 reports the received noise power in 2kHz bandwidth using a lossless antenna at VK1OD. The graph suggests that ambient noise is very rarely below -110dBm in 2kHz bandwidth, or -143dBm/Hz Noise Power Density (NPD).
The NPD of a resistor at room temperature is -174dBm/Hz, so the receiver Noise Figure for internal noise to be at least 10dB below external noise must be less than -143-(-174)-10=21dB. That should not be too hard to achieve, good HF comms receivers are usually have Noise Figure lower than 10dB.
Receiver requirements were:
A Codan 7004 receiver with ovenised crystal met the requirements and was was available for re-purposing.
The 7004 is a compact crystal locked receiver with specification Noise Figure of 7dB. It requires selected capacitors through the front end and oscillator unit to align it for a given frequency. The configuration used required a crystal at 8688.6 to receive USB with virtual carrier at 7038.6kHz. All Tantalum caps were replaced as they are a potential source of problems, and on advice, particularly in the AGC circuit in this receiver.
The receiver draws about 0.2A at 13.8VDC, current depends a little on ambient temperature due to the varying requirements of the crystal oven. Observation suggests that frequency error should be less than 5Hz in operation.
After alignment, the receiver measured a sensitivity of -124dBm for 10dB SINAD, which with ENB of 2750Hz implies a Noise Figure of 6.5dB (calculated with Receiver sensitivity metric converter). Gain compression of 1dB occurs at -100dBm input, so AGC action is delayed until about 33dB above the receiver noise floor. AGC characteristics are for SSB telephony, and benefit may be obtained from reducing the decay time a little (this is an early version rx and does not have the AGC links that are in the later version).
Working back from the receiver Noise Figure, for internal noise to be at least 10dB below external noise, the average antenna gain must be at least -(-143-(-174)-6.5-10)=-14.5dBi, so a shortened loaded vertical could easily satisfy. The test of adequate antenna gain is that receiver noise output increases by at least 15dB (which includes the ambient variance allowance) when switching from a 50Ω termination to the antenna.
Fig 3 above shows the measured IF amplitude frequency response. The response was measured by plotting the receiver audio spectral output with a wide band noise source input. A small spike from hum appears in the lower part of the curve, this will be a measurement artifact rather than a spike in the filter response. The filter has quite a flat inband response. The Effective Noise Bandwidth was calculated from the spectrum plot, and it is 2750Hz using 1kHz as the gain datum. WSPR uses a 200Hz segment centred on 1500Hz.
The Jones plug was removed and replaced with a small aluminium plate with 2.1mm DC connector for power and a 3.5mm stereo jack for the line out as shown above.
This receiver's remote RF gain board and some unused wires were removed to de-congest the area around the rear sockets. Fig 5 shows the revised internal wiring to the power and line out jacks. The Jones plug was removed and replaced with a small aluminium plate with 2.1mm DC connector for power and a 3.5mm stereo jack for the line out as shown above.
An on air test for 24 hours verified system performance. Comparison of logged S/N ratios with adjacent stations receiving the same DX stations confirms the low ambient noise levels at VK1OD. The antenna used is a low half wave inverted V dipole, see Bowral 7MHz dipole for local contacts for more detail.
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