Last updated: Sept 16 2027
Log:
added [Section 1] Band-pass Filter
We'll slowly add content over time and log new additions. We imagine that some sections will get their own web page -- linked from this page. Thank you.
[Section 1] Band-pass Filter
In this section we write about making a simple BP filter for our 40 meter band direct conversion (DC) radio. After the BNC jack antenna input lies a DPDT switch to open or throw a 10 dB, 50 Ω attenuator pad in-situ.
Thinking about our wonderful global audience, some builders - including us - use Linux or FreeBSD and try to run free -- and whenever possible, open-source tools that were written or distributed by moral people/companies. Thus, we employed Marki Microwaves filter designer program on a FreeBSD OS laden bench laptop. We never seek to prescribe --- please use whatever design tools on whichever computer OS you prefer. You’re in control.
For both antenna, filter and receiver experiments, we’ll centre our filter at 7.20 MHz
The 4 main factors that influenced this filter design
1. Since this doubled-tuned, top-coupled bandpass affair input filters a DC receiver, we’re not concerned with image stripping. True, the high-pass filter skirt runs steeper than the low-pass skirt, but does it really matter for a simple 7 MHz DC radio? Perhaps it does for your design. You might seek a triple-tuned filter or other network. Indeed - we prefer that you design your own filter with intention – and sincerely hope you enjoy this work.
2. As mentioned – we used the free Marki microwave software that
works in most web browsers.
Link to Tool Aside from working independent of the users preferred OS, we trust
this software – since, historically, actual built circuits measured
similarly to the calculated values & simulations provided by the online Marki microwave
software.
3. We sought low insertion loss because we normally use electrically short antennas. To us low insertion loss is < 2 dB for a bandpass filter.
[Reference 1] The Double-Tuned Circuit: An Experimenter's Tutorial by Wes, W7ZOI in QST for December 1991. An online version is linked below:
From this reference we chose a 290 KHz -3 dB bandwidth – Roughly calculating: 7.20 MHz/ 0.290 MHz = a Qt of 24.8 . If the unloaded Q of each inductor = 250, then 250 /24.8 gives a normalised Q of 10. For a normalised Q of 10, we know that an insertion loss of < 2 dB is possible.
4. Finally, we sought a simple design that uses standard value 5% capacitors that we stock.
Making the Filter
Measuring inductance seems superior to using a toroid wire turns calculator. We made 2 coils wound on T50-2 iron dust toroids and measured them close to 3 µH.
The filter was peaked using a 50 Ω signal generator set to 7.20 MHz driving a 50 Ω input Z DSO. We ensured that the total capacitance [120 p + CV] in each tank provided a sufficient range to properly tune each tank across the 40 meter amateur band. 1 way to do this – before soldering in the 120 pF + CV caps, place a ~10-150 pF or so variable capacitor in a tank circuit and measure the capacitance after signal peaking -and then remove this temporary capacitor.
At 7.20 MHz, the temporary variable cap measured 123.3 pF. So a 120 pF fixed value cap plus CV would seemingly work OK.
Then our filter went in a tracking generator/spectrum analyser for a sweep.
Above — Sweep 1. Insertion loss ranks on target at under 2 dB. No double-humped response. Works as designed.
Return loss (RL): At 7.2 MHz - with the other end terminated in a 50 Ω resistance, the RL measured 18.7 dB. With the other end terminated in 100 Ω, the return loss degraded to 15.6 dB. Switching in the 10 dB pad on the other end with a 100 Ω resistance returned the RL to 18.7 dB.
We removed the 47 pF 5% coupling capacitor and substituted a QuL = 1000 air variable capacitor and fine tuned for the best possible return loss with the other end terminated in a 50 Ω resistance. RL = 23.9 dB. We removed this air variable capacitor and measured it at 46.9 pF. Wow Marki microwave!
The removed 5% tolerance 47 pF fixed capacitor measured 48.6 pF – and we soldered it back in place. Alas, we have to work with standard value parts with tolerances. Some builders use 10% capacitors – real life on the bench.
Above — Sweep 2. Zoomed in sweep span. While no panacea, this so simple filter should work OK. All the capacitors were good quality parts with C0G temp coefficients. The small range variable caps facilitated precise tuning of each tank.
Conclusion
A basic filter with a measured insertion loss of < 2
dB was designed, built & validated with intention.
| -------------------------------------- end-of-section-1 ------------------------------------------|