HF Amateur Bands Preselector (Built & Tested PCB Module with aligned inductors)
The Module includes a high-quality ENIG FR4 PCB (180 x 50 mm) that comes with all components factory pre-soldered.
RF grade capacitors are used for all the circuits. High Q inductors and RF type relays are used.
Six SMA female connectors are also soldered in place.
Provision for external pre-amp and attenuator control
9-bands (160m - 10m) and by-pass function
RF relay switched band-pass filters with DC wetting
Building a multi-band receiver for HF? Or an HF up-converter for SDR?
Then this HF Ham-Bands Pre-Selector Module can be used to enhance reception performance on all amateur radio frequency bands by significantly reducing the signal strength of most unwanted signals outside those frequency bands and greatly improve IMD response. If you have never used a filter before your HF upconverter for SDR then you will immediately see that it greatly improves receiver performance on the wanted frequency band by rejecting most unwanted.
The design uses RF type 5V DC relays to activate one band-pass filter every time, as selected by the control connector pins.
Each band-pass filter uses 2 variable shielded inductors and 3 capacitors. You can see their frequency-gain response below, after proper inductor alignment.
For even better results you can easily cascade two of those modules and use one HF pre-amp between them.
Except of selecting one of the 9 band-pass filters, one for every amateur radio bands for HF, a by-pass function is available to switch out the preselector, if reception to any out of bands signal is needed.
In addition, any external fixed or variable attenuator can be switched in or out at the input, using an on-board relay and SMA connectors and any external pre-amplifier can also be switched in or out of the circuit after the filters.
Hand-operated selector type switches can be used and any type of micro-controller interface, as 2N7002 MOSFETS are used for driving the relays.
The new design has pull-down resistors installed on the PCB.
The relays are powered from a 5V DC input, while the selection is done by applying a 3.3V (or 5V DC) - TTL "HIGH" to each MOSFET's gate. Every relay needs about 30-50 mA so the total current per module for the 5V supply can be 150 mA max if the Attenuator and Pre-Amp relays are energized altogether.