Hardware · Discipline
RF & Wireless
Getting a receive antenna away from the rack without spending the link budget on cable.
Radio microphone problems are almost never about the radio. They are about where the antenna is, what is between it and the receiver, and how much of the system's sensitivity was given away in a coax run down the side of a stage.
This is one board family so far, and it is the physical-layer half of work that already exists here as software: the coordination, monitoring and licensing tools are on the software side, and they assume the antenna is somewhere useful. This is how it gets there.
1 board · none fabricated yet
RF & wireless · Antenna pair over one Cat cable
UHF Pair Extender
Front-end board placed · routing next
Remotes a wireless-mic receive antenna pair over a single shielded Cat cable. Filtering, gain and line drivers at the antenna end; balun, slope equalisation and a matched step attenuator pair at the rack end. 470-698 MHz, A/B diversity preserved, one cable instead of two coax runs down the side of a stage.
- Diversity stays diversityEach antenna gets its own individually screened pair. With a single overall screen, pair-to-pair crosstalk at 500 MHz is 30-40 dB, which couples antenna A into B and destroys the independence the receiver's diversity logic depends on - silently.
- Noise figure is the whole design0.97 dB system noise figure at any attenuator setting, because the step attenuator sits after the line driver rather than mid-chain. The same parts mid-chain give 11.8 dB.
- The preselector is split on purposeLow-loss filtering before the first LNA, where its 0.46 dB lands directly on noise figure; the lossy LTE traps after 20 dB of gain, where 3.3 dB costs a hundredth as much. Worth 2.8 dB of sensitivity for no extra parts.
- No AGC, everReceivers derive squelch thresholds and diversity decisions from absolute RF level. Gain is measured once at commissioning and then frozen. A running loop that holds a target level is a defect here, not a feature.
- Power gets its own pairWideband UHF transformers rate around 30 mA and the head needs roughly 200 mA, so the DC feed rides the one pair that is split at the connector and therefore useless for signal anyway. Never parallel two pairs for one RF signal: cable delay skew puts comb nulls inside the passband.
Honest status
Three boards, and the one being built first is not a product. The front-end characterisation board is a single-channel test board that exists to decide whether either production board works, because it can be measured with a VNA and one receiver - its schematic is ERC-clean and all 43 footprints are placed, but it is not routed and nothing has been fabricated. The head unit's schematics are roughly 80% drawn; the rack unit is an empty project. Every figure quoted above is a budget computed from datasheet numbers, not a measurement, and the point of the first board is to find out which of them are true.
- Band
- 470-698 MHz, A/B diversity
- Cable
- One S/FTP Cat run: 2 RF pairs, 1 DC, 1 RS-485
- Front end
- Split preselector either side of the first LNA
- Gain block
- QPL9547 LNA, TCM2-33WX+ balun
- Level control
- PE4312 step attenuator, both channels one control word
- Boards
- Head, rack, and a front-end characterisation board
- Stack-up
- 4-layer, impedance controlled, 50 ohm microstrip
- Repo
- Private
Where the sensitivity actually goes
Three decisions in a remote antenna head account for almost all of its performance, and all three are counter-intuitive enough to be worth stating. Loss before the first amplifier lands directly on noise figure, so the low-loss preselector goes first and the lossy LTE traps go after 20 dB of gain, where they cost a hundredth as much — worth 2.8 dB of sensitivity for no extra parts. The step attenuator goes after the driver, not mid-chain: same parts, 0.97 dB system noise figure instead of 11.8 dB.
And the third is a negative: no AGC, ever. Receivers derive squelch thresholds and diversity decisions from absolute RF level, so gain is measured once at commissioning and then frozen. A loop that quietly holds a target level is a defect here, not a feature — and it is the kind of defect that presents as an intermittent dropout three weeks later.
Diversity has to stay diverse
Two antennas sharing one cable is only worth doing if the two paths stay independent. With a single overall screen, pair-to-pair crosstalk at 500 MHz is 30–40 dB, which couples antenna A into antenna B and destroys the independence the receiver's diversity logic is built on — without ever announcing itself. So the cable specification is individually screened pairs, and it is a requirement rather than a recommendation.
The related trap is paralleling two pairs to carry one signal. Pairs are twisted at different rates by design, so they differ in electrical length; the allowed skew puts comb nulls inside the passband at UHF. It is a digital problem dressed as an analogue shortcut.
Related, elsewhere on the site
The whole bench
This is one discipline of several
The hardware index carries every board at once, filterable by discipline — lighting nodes, audio boxes, video processing, the physical layer under all of it, and the RF work above it.