The recommended Prescaler is ridiculously simple. It consists of just one IC,
a TV tuner prescaler, the Philips SAB6456A, which can divide by 64 or by 256.
This chip is widely available both new and in the surplus market at much lower
prices than conventional divide by 10 prescalers.
Prescaler layout (left) and schematic (right), about life size
(Click on image for enlarged view)
Looking at the schematic, to the right, the chip is an ECL device with balanced
input and output,
but for our purposes works quite well single ended. The signal is coupled to one
input, pin 2, while the other input, pin 3, is decoupled to ground. Input biassing occurrs
internally. Pin 5, the mode pin, is left open to select divide by 64 mode. Pin 6
is one of the outputs, and is also unused. Pin 7 is used as the output and is AC
coupled. The supply is decoupled and connected to pin 8.
The device sensitivity is about 10mV RMS over the range 70 - 1000 Mhz, and the output is
typically 1V p-p. The input resistance varies from 560 down to 30 Ohms, and the input
capacitance, excluding the PCB, no more than 5pF.
The layout is shown in the left diagram. The circuit is constructed on a small
piece of single sided PCB, with the copper side up. The grey areas are copper,
while the white areas have the copper removed, either by milling, cutting or etching.
The board is very small - about 25mm high and 30mm wide. The chip is "operated on"
before placement. Pins 1, 5 and 6 are cut very short, and the remaining pins spread out flat.
The chip is placed right-side up (label up) and pressed down so the remaining pins
flatten out on the board. Solder pins 4 and 8 to the copper below, which will hold
the device in place. Prise pins 2, 3 and 7 up so they are parallel to the board,
but not touching. Trim these leads shorter, but leaving enough pin to solder to.
Solder a capacitor to each of these pins (with minimum possible lead length)
and solder the other end to the board. The black blobs represent solder. Finally
solder capacitor C3 between the two copper lands close to the chip near pin 8.
In all cases the IC leads and capacitor leads should be kept as short as possible.
Best performance will be achieved using zero lead length techniques and surface
mount capacitors.
To build the Prescaler into your ZL1BPU Counter, you will need an extra BNC connector
and a DPST switch for the front panel. Wire the BNC connector to the Prescaler input
(preferably mount the Prescaler on the back of the connector). Use one pole of the
switch to connect the Prescaler output to the Counter input, and the other pole to
apply +5V power to the Prescaler. If you really want to get fancy, use a 3PST switch,
and use the third pole to pull PB5 on the micro to ground, which will force it into
64 Prescaler mode. Alternatively, use a multi-pole Mode switch which will apply
power and switch the Prescaler output.