Dual RF Head

Dual RF Head sensor

I needed something to measure low RF levels, primarily from directional couplers etc. Currently I'm building a 500W PA for 70 cm (based on W6PQL module), on the output is a Low Pass Filter which has a directional coupler built in. This coupler has the usual diode so it will give an DC voltage out for both forward and reflected power.
I choose to disconnect the diode circuit, and bring the RF directly out (coupling approx -40 dB).
This created a need for this "Dual RF Head" sensor.

I then went a little overboard and made it perhaps a bit more "professional" than needed, but you know: "why make it simple when you can make it complicated" :)

Background

I based the design on a dual logarithmic sensor chip, the ADL5519 from Analog Devices. The chip is not exactly "cheap", but it has some very nice features, primarily:


  • Wide bandwidth: 1 MHz to 10 GHz
  • Dual-channel and channel difference output ports
  • Integrated accurate scaled temperature sensor
  • 62 dB dynamic range (±3 dB)
  • >50 dB with ±1 dB up to 8 GHz


Datasheet for ADL5519


I added a 10 dB attenuator on both inputs, this ensures a better return loss on the two inputs. I also added two opamps to give a little bit of gain to the output signal. The ADL5519 delivers approx 1.7V with no RF input, the opamps scales this to around 4.25V (multiplies the output from ADL5519 with 2.5). As the sensor will be connected to my "REPAM" device that has 0..5V analog inputs, the addition of the opamps made sense.

The design

The PCB board is made on double sided FR4, 0.8 mm thick. I did not strive for anything spectacular with regards to RF precision, the sensor will be calibrated for whatever frequency range it will be used at anyway.

The sensor is supplied with 8 to 14V DC (nominal 12V, absolute maximum 16V) and consumes around 50 mA.

There are 4 output signals, OUT A/B, these are the logarithmic outputs from the two channels, A and B. The output is close to -55 mV/dB. No RF in will give approx 4.25V output, +10 dBm will give approx 1.6V.

The OUT P/N are outputs directly from the ADL5519, these are the sum and difference outputs. You can use one of these for detecting high SWR etc.

The REPAM box has settings for calibration factors etc for the sensor, it will also monitor forward, reflected and swr, and enable alarms, setting of relay outputs etc when thresholds are exceeded.


Box design

I designed a small aluminum housing for the board, this makes the board somewhat RF "tight" (not completely true because of the plugin screw connector).

I used JLCPCB's 3D milling service to make the box (total cost €18,- for a single one, if ordering 5 pcs, price each is only €8.80 !). The surface is conductive anodizing. All screws are M2.5, the box also holds two SMA female bulkhead connectors.


Frontplate

Many times I design frontplates in PCB material. This has a number of obvious benefits. For the Dual RF Head sensor I made the frontplate in 0.8mm PCB material, black with white silkscreen.

Returnloss.

First the returnloss is measured for both port A and port B. As my VNA (R&S ZNLE3) only covers up to 3 GHz, I did not measure at higher frequencies than that

Calibration

The Dual RF Head device can be used with my REPAM unit, below is the configuration data for the Dual RF Head when connected to the directional coupler in a W6PQL LPF module for 70 cm. The directional coupler had its diode bypassed (the series capacitor was just removed), the output for channel A (FWD) and channel B (REF) was taken at the attenuators in the coupler (the attenuation factors was set to -20 dB for FWD and -13 dB for REF)

More info coming....