When investigating the equipment requirements for microwave EME, it soon became clear that operating from the shack, 140’ from the dish was out of the question. For this reason, the EME shack would be in the garage, just 50’ from the dish. While on 1296MHz the loss of the transmit feeder was tolerable, it was decided the only solution on 2304MHz and above was to put the amplifier at the dish. A Spectrian 200W amplifier was obtained for 13cm converted for 28V operation following the instructions of N5AC (1) It was put out at the dish and this arrangement resulted in successful QSOS. Subsequent experiences, mainly incomplete QSOs due to no output power, revealed that it was vital to have a means of monitoring the output of the amplifier from the operating position.
Initially the signal from the sample port on the front panel of the amplifier, which provides a –50dB sample, was fed through a piece of LDF2 back to the shack to be displayed on an HP436A. This was cumbersome. Thoughts moved to rectifying this sample and feeding back dc to the shack, but this had no advantages over the previous method. At this point I realised that as the amplifier was used in commercial operations, it must be monitoring its health, so there had to be check points. While replacing a melted output combiner board I noticed that there was a 6pin connector connected to what appeared to be detector circuits. Investigation revealed this had a forward power dc voltage, a reverse power DC voltage, and supply volts (to power the detectors) and ground as follows:-
|
Pin |
Function |
|
1 |
Forward Power |
|
2 |
Reverse Power |
|
3 |
8V |
|
4,5,6 |
Ground |
I wanted to check the calibration of the forward power voltage. To do this the voltage at pin 1 of the 6 pin connector was measured against forward output power as measured on an HP power meter:-
I decided it would also be nice to monitor the current taken by the amplifier remotely. I had already built an HF amplifier which required high current monitoring which was achieved with a design by Paul Wade (2).The original design produced a 5V output when the current was 50A. By changing the sensor to an Allegro ACS750SCA-100 (3), available from Digikey, 5V is produced when 100A is taken. This voltage could be fed back to the shack.
One point noticed when using the amplifier on 28V was that at full 200W output the circuit breaker would trip after a short period of key down. This is unacceptable for moonbounce operation. Removing the breaker from the chassis after removing the 8 front panel mounting screws, the label showed it tripped at 37.5A after a short delay. This is an appropriate current when the amplifier is running from the originally specified 48V but at 28V where about twice the current will be needed will cause problems. I did investigate a source of replacement breakers, but the higher current sensors have a different form factor and terminals and would not be a straight replacement. For this reason the original circuit breaker was removed and the current monitoring board mounted in the resultant space. To protect the amplifier the current limiting feature of the 28V Power supply was used.
It would be nice to be able the temperature of the unit in the shack. All three final amplifier modules and the 10W driver have integral temperature sensors which produce 10mV/degree F. It would be possible to connect a wire to these sensors and send it out of the unit, but introducing extra wires in the PA compartment may introduce instability. For this reason time was taken to trace the signals to the PSU side of the chassis ending at J4, which is mounted on the power distribution board. The findings are as follows:-
|
Module |
Module
Pin |
J4
Pin |
|
Driver |
9 |
20 |
|
Amp
1 |
1 |
22 |
|
Amp
2 |
1 |
29 |
|
Amp
3 |
1 |
32 |
As it was not necessary to monitor the temperature of all modules only the centre PA module Voltage is fed to the shack (J4 pin 29)
The next decision was how to interface these voltages to the outside world. Although it would be possible to feed the voltages directly to the out of the unit, any issues with the cable or display could damage the Spectrian unit. For this reason the voltages are fed through voltage followers, implemented with a quad single rail opamp type LM324. The worse that could happen is the chip would be sacrificed and cost $1 to replace. The opamp was powered with a 7812 regulator. Although the amplifier has 12V internally available, any potential damage to the amplifier would be avoided by providing a 12V supply from a separate regulator. The 12V is also fed to the remote indicator.
Initially the shack display uses a 3.5 digit LCD voltmeter module set for 20V FSD. It is configured to run off 5V, supplied by a 7805, with a common input/measurement ground. A 1P4W rotary switch is used to select which parameter is displayed. The circuit of both modules is shown below.
Construction
The
Current monitoring circuit was built on a 2.5 x 1.5” PCB available from W1GHZ. This is mounted in
the space vacated by the circuit breaker. The LM324 was mounted in a socket on
a piece of stripboard, mounted near the back panel. The 7812 regulator was
mounted on the chassis. The monitoring signals are provided on a 6 pin minidin connector
chosen for no other reason that the
local electronics emporium had 50’ premade 6pin minidin cables available. The
pin designations are as follows:-
|
Pin |
Voltage |
|
1 |
Reflected
power |
|
2 |
Forward
power |
|
3 |
12v |
|
4 |
Ground
on transmit |
|
5 |
Current
monitor |
|
6 |
Temperature |
The display is mounted in a 5x2x2” aluminum box with the switch and connector mounted on the back panel.
Hopefully these notes will allow those who need to remotely mount their Spectrian Amplifiers to monitor the health of the unit.
Work is currently under way to use a PIC and 4 line LCD display to give a method of monitoring all parameters simultaneously, like as is seen on some high end HF amplifiers.
;References