Some early observations


I’ve taken the opportunity to make some measurements and comment on some on-the-air observations to see how the new FCD+ weighs in.

The noise figure on 2m and 70cm is pretty much the same as on the original FCD, at about 3.5dB. In production, I measure for 0.15uV for 12dB SINAD in NBFM because that’s much easier to measure. However I also took the opportunity to set up a noise figure measurement mentioned to see how that measured up. On 6m, the noise figure’s 2.5dB. At 23cm, I measured a 5.5dB noise figure. I used VK1OD’s NFM software in conjunction with an RFD 2305 noise source to make these measurements.

Although the TCXO reference oscillator (26MHz) specification says it’s 0.5ppm I get more like 1.5ppm when I compare it with the rubidium source on the bench. Either way, it beats the original FCD hands-down.

Comparing the receiver blocking experienced by some on the FCD at 2m, this is like night-and-day on the FCD+. Interference previously experienced that was caused by VHF broadcast (and VHF pager transmissions here in the UK) has disappeared. This is due to a number of factors, including a new LNA, 3.15V RF section and the front-end filtering.

Despite having some pretty strong local FM broadcast transmissions, airband reception just above the FM broadcast band appears unaffected, even though the same bandpass filter is used. Again, this is due to the improved LNA performance and the increased headroom in the RF section.

VHF wideband FM is super crisp and clear, no longer distorted, due to the 192kHz sampling rate.

Reception below 32MHz does not use the LNA, and goes through various specially designed passive bandpass filters. Comparing with other MF/HF receivers, the SNR seems on a par. I have not yet experienced any overloading myself on MF/HF.

On LF/long wave, the FCD+ appears a bit on the deaf side, although I’ve had no difficulty picking up RTE Radio 1 on 252kHz or Radio 4 on 198kHz with an HVU-8 antenna sat outside my kitchen window. I was unable to pick up the MSF time signal that another receiver I have here could just about resolve, but that is down at 60kHz, so out of the FCD+ specification anyway.

When receiving with a sampling rate of 192kHz with no antenna connected, there is a noise “blob” at about +/-86kHz, about 10dB above the noise floor. Interestingly the ADC manufacturer also shows this on one of their application notes. It wonders around a couple of kHz over time. It’s not clear what this is, perhaps an on-chip LDO or other votage converter, perhaps for its PLL. Once an antenna is connected, the “blob” is under the noise floor so it’ll usually be of little consequence in practice.

Many thanks, Howard

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