8-Band RF Output Switch
For those of you who want to drive Microwave transverters from your Portsdown or need a higher drive level, the team have developed an 8-Band RF output switch that includes an RF amplifier. It is designed to be used with the Output Attenuator (see below). It will be capable of amplifying the variable level from the attenuator providing up to 10 dBm and switching it to separate amplifiers for the 70, 144, 437 and 1255 MHz bands, and to 4 transverters for the higher bands. PCBs are available from the BATC Shop.
The initial design was as shown above - the first batch of boards will be supplied in this form. The initial (V1.0) circuit is here: Media:8 BAND RF SWITCH V1-0 SCH.pdf. The first board layout is here: Media:8 BAND RF SWITCH V1-0 SS.pdf.
The original parts list, which has not yet been updated, and has only been checked against the DigiKey Part Numbers, is here: File:8 BAND RF SWITCH V1-0 PARTS.xls. This parts list does not yet include components for the input attenuator.
HOWEVER, the circuit needed some simple rework to provide a more stable power supply for the SKY MMIC and to equalise the levels across the bands. The modifications are to:
- Cut the track on the underside between the output of the voltage regulator and the link point marked 5VA.
- Replace the 7805 with a 7808 regulator (for 8 volts) and wire a 30 ohm resistor between the regulator output and the the point marked 5VB. Note that the resistor will need to dissipate 300mW (it passes 100mA), so 3 1/8 watt 10 ohm resistors in series might be easiest.
- Wire a new 78L05 regulator on the underside of the board with its input from the 12V supply and its output to the point marked 5VA.
- The input attenuator should be set up with R1=R2=68 ohms, and R3=220 ohms. To get the full 10 dBm output on 1255 MHz, add a series combination of 68 ohms and 2.7 pf across R3. The first batch of PCBs do not have pads for the extra capacitor and resistor.
The modified board is shown below.
Typical Portsdown output levels with the digital attenuator in-circuit and set to 0 dB and then 3 dB are shown below.
| Freq | Level @0dB Atten | Shoulders | Level @-3dB | Shoulders |
|---|---|---|---|---|
| 71 | +10 dBm | -45 dB | +7 dBm | -50 dB |
| 146 | +12 dBm | -45 dB | +9 dBm | -50 dB |
| 437 | +14.5 dBm | -45 dB | +11.5 dBm | -50 dB |
| 1255 | +11 dBm | -45 dB | +8 dBm | -45 dB |
The new circuit is here: Media:8 BAND RF SWITCH V2-1 SCH.pdf