Termination impedance of 9MHz IF filter - Page 1
Forum section: electronics / rf-microwave

author: quantumvolt
author_rank: Frequent Contributor
timestamp: June 19, 2016, 08:01:46 am
content: I am going to change IF in a HF receiver from 8.9875 to 9.0 MHz. The wide monolithic filter XF01 in the schematic below (Yaesu FT-77) will be replaced with a homebrew narrow crystal filter for CW reception only.My first question is if I need to build the new filter with specific in and out termination impedances, and if so - how can I determine the impedances for the existing filter from the filter itself or/and the circuitry before and after it.Nothing is known about the filter except from "8F-20A/S, Monolithic Filter, RX IF Filter" - see page 17 in:www.yaesu.com/jp/manuals/yaesu_m/FT-77_tebiki_J.pdfThe full schematic for the RF Unit is on page 22. From other Yaesu filter terminology I guess the original filter is a 20 kHz AM (/SSB?) filter.

author: Fank1
author_rank: Regular Contributor
timestamp: June 19, 2016, 11:36:33 am
content: As a "wild guess":Most filers have the same input and output impedances.The dual gate amplifier following the filter would have a very high input impedance.It is shunted by the 3.3K resistor, so I would guess about 3K for the filter impedance.

author: G0HZU
author_rank: Super Contributor
timestamp: June 19, 2016, 12:19:08 pm
content: My wild guess would be that it could be an odd filter with a low Z 50R input and it wants to terminate into ~3k? on the output in parallel with a few pF.So maybe 50R input and 3k? // -5pF output?Also, you need to provide a DC path across the input pins to allow the correct bias control of D41 etc. This would normally be done via the windings of an input transformer inside the xtal filter.

author: quantumvolt
author_rank: Frequent Contributor
timestamp: June 19, 2016, 12:26:22 pm
content: I have searched a bit on the net and found that most IF filters have termination impedance of some 50 to 1000 ohm. The impedance can be transformed with an inductor or transformer before and/or after the filter. Software exists for the design calculations.I agree with Fank1 regarding the output side. C62 0.01uF and C63 0.022uF (by default) should represent little impedance at 9MHz, so the termination load is 3300 ohm. But on the input side the filter is in a low impedance loop formed by C59 0.022uF in series with with secondary of T20, D41 PIN diode and C61 0.022uF. And the fact that the so called 'monolithic' filter has 4 terminals could mean it has different termination impedance in and out.I do not know but I have decided I will learn. But of course, in the end I can just ignore the impedance question and try to measure or listen to the power transfer / loss by injecting 1-50uV at the antenna input. All I will experience (I think) is some ripple and asymmetry in the pass band.

author: quantumvolt
author_rank: Frequent Contributor
timestamp: June 19, 2016, 12:33:44 pm
content: G0HSU, you posted while I was writing. I think you are right. R57 470 ohm is RX 13.5 volt through the filter, the PIN diode, the secondary of T20 and to GND via R55 470 ohm.Is there a way to measure anything to confirm or discard the two-impedance hypothesis?

author: G0HZU
author_rank: Super Contributor
timestamp: June 19, 2016, 12:45:45 pm
content: Yes, if you look at the input side there is a mixer Q05 and if I assume it is symmetrical via the input and output transformers T19 and T20 (do these two transformers have the same part number?)then there is a useful clue from the LPF formed by L12. L12 is located just above the mixer Q05.A LPF made from a pair of 82p caps and a 0.33uH inductor will have low ripple and a cutoff frequency of just over 30MHz if it is designed for 50R impedance. So this may be a clue that the filter is 50R input impedance because mixer Q05 'probably' has 50R input and output impedance?

author: quantumvolt
author_rank: Frequent Contributor
timestamp: June 19, 2016, 01:29:11 pm
content: Yes, the 2 mixer transformers T19, T20 have the same part number (T1019, 20 page 43 inwww.radiomanual.info/schemi/FT77_user.pdf).The LPF observation C56, L12, C57 solves the input termination impedance. Very smart. G0HZU

author: quantumvolt
author_rank: Frequent Contributor
timestamp: June 19, 2016, 03:40:16 pm
content: Well, thanks a lot to Fank1 and G0HZU.I searched for impedance matching of 4-terminal monolithic filters, and I guess it is quite a bit over me for now. A model of the filter in discrete component topology is inhttp://www.radio-electronics.com/info/rf-technology-design/receiver-selectivity/monolithic-crystal-filters.php.In practice, however, it turns out that the filter is characterized by four 'composite' parameters R1, C1, R2, C2 always given by the manufacturer of the 'raw' monolithic crystal filter. Based on these the filter impedance is matched with a network to 50 ohm or whatever as shown inwww.vectron.com/products/saw/pdf_mqf/TECHINFO.pdf. This matching is already done by Yaesu for the input side only - the filter can includes an input parallel inductor Lp that both takes part in matching input termination impedance and allows DC current to flow through the filter input (see the last link - 2nd case: Ro less than R1 - for details and serious math). Trying to build a filter with different input and output characteristics seems somewhat special to me.Since my IF/AF chain is from an old Icom, I will have to change from a 9.0115 MHz / 15 kHz FL-23 filter to a narrow 9.0 MHz filter there also. More uncertainty.I therefore think I will tap (divert by removing D41 and D42) the 50 ohm signal after the Yaesu mixer. Similarly I enter the ICOM IF path after the last IF filter and only use the last IF amplifier, the product detector and the AF amplifier. I can then use a 50 ohm input / ouput impedance filter that I will find tutorials for in several ham related web sites. If necessary with a DIY 50 ohm IF amp module somewhere in between.As a bonus I will not have to integrate the Icom AGC voltage into the Yaesu last RF amplifier. I prefer to listen with AGC off and a hand on a RF Gain attenuator pot anyway.Problem solved. No wonder hams and RF-nerds build 50 ohm modules.Thanks again for the help to G0HZU and Fank1.