Required power for AUTO Band sensing

[WJ6A]

I was surprised that this is not in the manual I can find that you need at least 2 watts or more input for the PA100-D to sense and change to the correct band. This is OK with me just good to know. I will be using the Juma PA100-D on "G1" because I will be using it, if possible as a intermediate amp to drive my Mercury IIIs 1200-watt amplifier. My question is AFTER the PA100-D has properly switched to correct band, and you present it with a 1:1 SWR, looks like 1.6 watts will give you and output of around 34 watts, can you lower the input more to the amp if you want less output. Has anyone checked this and would it cause harm. Thanks, WJ6A

[5B4AIY]

Hi,

I'm surprised that you think your amplifier needs at least 2 watts for the Frequency Sense logic to operate. I'm not sure which firmware you are using, but with my firmware the frequency sense logic will operate with as little as 100mW.

Second, the design of the amplifier is such that the average power gain is as follows:

G1 9dB
G2 11dB
G3 13dB
G4 15dB

The 'gain' is actually selecting which of two attenuators are in circuit. If you examine the schematic I have attached to this post it shows that there are two attenuators, a 2dB pad and a 4dB pad. Relays RL1, RL2 and RL3 govern whether the drive signal is routed directly to the PA, or whether a 2dB, 4dB or the combined 2+4dB pads are inserted.

With these gain settings, using +30dBm / 1W drive signal will give the following nominal output powers:

G1 39dBm 8W
G2 41dBm 12.5W
G3 43dBm 20W
G4 45dBm 31.6W

Since the amplifier is a linear device, the output power will be proportional to the input power up to the 1dB gain compression point. Any further increase in input drive will simply eventually saturate the amplifier. Therefore, for best linearity you should operate the PA100D with as low an output power as possible. Equally, the power from the driver transceiver needs to be considered.

Although the best input match to the PA100D is obtained at the G1 setting, if you need about 30W, 45dBm output, then at the G1 gain setting you would need +36dBm or about 4W of drive signal. If you are using a 10W transceiver such as the Elecraft KX3/KX2, or Lab599 Discovery, Icom IC-705, then this would be a suitable operating point to minimise distortion.

However, if you are using a 5W transceiver then this is very close to its maximum output and its distortion products would be fairly high. In this case, I would suggest operating the PA100D using the G2 gain setting, whereupon you would only need +34dBm or 2.5W of drive signal, and a 5W transceiver should be able to give this with reasonably low distortion.

73, Adrian, 5B4AIY