Operating instructions

5.8 1W Broadband HF/VHF Power Amplifier (Sheet 8)

The RF output of Sheet 7 (VCO_OUT) becomes the primary input to this circuit

(RF_IN).

This RF input is first amplified to a level of about +2 to +4dBm by U800, then it is

amplified by Q801 to about +20dBm (with full bias), and then

it is amplified by Q804

and Q805 to +30dBm. The output stage gain is less above about 42MHz, so that the

peak output power falls to about +26dBm at 50MHz.

The effective gain of Q801 is controlled by adjusting the bias level (PWRCNTRL).

This can vary from 0.6V to nearly 6V, and is adjusted by a DAC output and a CPU

digital output, in Sheet 4.

The software monitors the forward power sense in the External Power

Amplifier(PA) as

well as the Reverse Power, temperatures, drain currents etc. It does this via the serial

bus (formed by SCLK and MOSI) and the select pin for the ADC converter on the

External PA (PA_CS).

The software then automatically adjusts the PWRCNTRL bias to:

1) Keep the forward power at the level defined by parameter PA_SET_FWD_PWR

(see 4.1), unless,

2) If the reverse power is > PA_SET_FWD_PWR*REV_PWR_ALARM/100.0

then the forward power is reduced until the reverse power stops exceeding this

limit, or,

The temperature of the PA output stage FETs exceeds 120C, in which case

forward power is reduced until this stops occurring.

5.9 Power Generation Section (Sheet 9)

There are three switch mode dc-dc converters in the board. These use monolithic

converters based on the National LM2595. Two of the converters are 12V converters

and one is a 5V converter.

The power in to the whole exciter is the voltage rail 28V.

U907 converts this down to 12V.

U908 is set up as an inverter, and uses the 12V rail to create –12V.

U909 converts the +12V rail to +5V for all the digital circuitry.

The +12V rail is used to power the on-board relay, as well as up to one extra off-board

relay. It is also dropped, via a linear regulator (U910) to produce the +10V rail, which

in turn is dropped by another linear regulator U911 to produce +5Q, which, in turn, is

dropped by a further linear regulator (U912) to produce +2.5V.

Similarly U913, U914, and U915 are linear regulators that produce –10V, -5V, and –

2.5V from the –12V output of U908.

+10, +5Q, and +2.5V, -10V, -5V, and –2.5V rails are used in the audio and RF sections.

D911 is a 4.096V (3%) reference diode. Its output is buffered by U906 which then

produces a reference voltage rail Vref, which is used by the CPU’s A/D converter, and

the DACs, and also in the voltage to current converters of the

VCAs (see Sheet 4, and

Sheet 7).