1. A Direct Conversion RX, DCRX, for the 40m band, with VFO input from an RF SIGGEN
2. An RF SIGGEN. Based on the Si5351 and covering 1-150MHz (my limits, the Si5351 can do more).
3. An AF SIGGEN. Based on the AD9833 and covering 0-100kHz (my limits, the AD9833 cam do more.
4. An RF METER which uses a -30dB TAP to allow power measurements. The TAP includes a 50R dummy load
5. A 12V/1.5A power supply to power the above
6. And finally, not yet boxed, an AF METER measuring in dBv and Vrms.
Also on top of the FDM-DUO transceiver is my MAT-125e auto ATU.
PROJECT
I use these along with my MacBook & software to measure the TX filter response of my ELAD FDM-DUO transceiver. This is the setup
Block diagram of the setup
The FDM-DUO transceiver has an RF output up to 5W over all bands. It has a USB audio input "TX" for digital transmissions. And a "CAT" serial data port which is an input and an output and is used with apps like WSJT for FT8 mode.
Connections on the FDM_DUO
The RF output is fed to the 30dB TAP attenuator, with a built-in 50R dummy load, and on to the RF METER which has an Arduino sketch written to display the power before the attenuator. This meter can display RF from -50 to +30dBm.
The USB CAT and TX audio inputs come from my MacBook. Where I use two applications, first a serial terminal app "iSerial" to connect to the FDM-DUO CAT port. This allows me to send PTT transmit and receive commands, "TX1;" ( 1 = USB Audio selected) and "RX;". The second app is Audio Hijack which is a wonderful app allowing you to make any audio connections inside the MacBook just by dragging the items to the screen. I have a USB audio ADC plugged into the USB input which is connected through to the FDM-DUO TX audio port:
The Hijack audio and iSerial terminal apps
The input to the USB ADC is from my AF SIGGEN I built a while ago (look down this blog for details, schematics and software). This SIGGEN covers 1-100kHz, but I need only 50-3000Hz for filter measurements.
To receive the FDM-DUO RF signals I am using a direct conversion receiver that I designed for the Banbury Amateur Radio Society "LEARN-CODE-BUILD" project that I ran a couple of years ago. It is a 40m DCRX with external VFO input from a matching RF SIGGEN. It was targeted for FT8 reception and performed excellently, allowing club members to become familiar with this digital mode using the WSJT app that few had tried at the time. The audio output of the DCRX goes finally to my new AF METER.
FILTER
To measure the filter response, set to the option of 200-2700Hz, the FDM-DUO is switched into transmit by sending a CAT command "TX1;" from the iSerial app. The AF SIGGEN output is adjusted to give 1-2W output power. The frequency is scanned in 50Hz steps over the low and high filter cut off limits and the AF signal from the DCRX is measured. This is the result
The cutoff frequencies of the FDM-DUO filter set to 200-2700Hz
As you can see the upper cut off is very sharp, which is good to avoid spill over to adjacent stations. The low cut off is a little smoother which gives fuller voice characteristic.
SDR WATERFALLS
It is very noticeable when using an SDR waterfall on your receiver, that you can see the spectrum of the other station. These vary a lot, showing both the Bandwidth and signal from full and solid across the frequency range to very narrow and bassy difficult to read signals or tinny thin signals.. It is also very clear when the station is using audio compression. We should I think give signal reports other than simple "5 & 9" by describing the waterfall specturm that is seen.
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