10.7 MHz IF / 30 kHz BW Resolution Filter

Overview

This is a 10.7 MHz narrowband (approximately 30 kHz) resolution filter for the GBPPR 1 GHz RF Spectrum Analyzer project.  This filter will allow the spectrum analzyer to display the RF signal(s) it's receiving in more detail than the stock 300 kHz filter.

The filter is made using four commericially available 10.7 MHz monolithic crystal filters from ECS.  These ECS-10.7-15B filters can be purchased from Digi-Key or Mouser and each "4-pole" package will include two matched filters, so you'll need to order two of these packages for a total of four crystals.  There is not enough stopband attenuation when using only two crystals and this can lead to spurious images on the analyzer's display.

Each of the crystals is marked with a black phasing dot on its case and this should be taken into account when making the final circuit.  The crystals have a input/output impedance of around 3,000 ohms, so a simple inductor/capacitor L-network will be used to transform this to 50 ohms.

Note that these particular crystal filters are not really designed for RF test equipment use, so they will have a fair amount of ripple in their passband.  This ripple can be adjusted slightly by adding a few low-value trimmer capacitors on the input/output of the crystals.

You can sometimes find commercial 10.7 MHz IF filters at hamfests.  Keep an eye out for these, as they will have much better passband and out-of-band attenuation characteristics.

Pictures & Construction Notes

Overview of the finished 10.7 MHz narrowband resolution filter.

It's built onto the lid of a salvaged Bud CU-124 case for ease of removal for modification or tuning.

Alternate view.

The input and outputs are symmetrical, and you can see the variable inductors (orange) used in the 3,000 ohm to 50 ohm matching section.

Little bits of copper PC board material are used to improve shielding between each of the crystals.

The RF connectors are TNC jacks.

Another view.

The two 2-10 pF trimmer capacitors can be used to slightly improve the filter's passband ripple characteristics.

Note the black phasing dots on the each of the crystals.

10.7 MHz narrowband resolution filter spectrum sweep.

Center displayed frequency is 10.7 MHz.

The settings are 10 dB per vertical division and 5 kHz per horizontal division.

Note the large amount of ripple in the passband.

The final passband may vary slightly with each crystal.

10.7 MHz narrowband resolution filter spectrum sweep.

Center displayed frequency is 10.7 MHz.

The settings are 10 dB per vertical division and 20 kHz per horizontal division.

10.7 MHz narrowband resolution filter spectrum sweep.

Center displayed frequency is 10.7 MHz.

The settings are 10 dB per vertical division and 1 MHz per horizontal division.

Note the fairly poor out-of-band rejection above 10.7 MHz, probably leakage from the impedance matching sections.

The final tuned filter has an insertion loss of around 9 dB.  You'll want to add a resistive attenuator to the other filter(s) used in your spectrum analyzer so they all have close to the same insertion loss.

View of the out-of-band peak at around 14 MHz.

10.7 MHz is on the left.

The settings are 10 dB per vertical division and 1 MHz per horizontal division.