What I would like to do is make a note of the ADC dBFS level with and without various filters. This will help define the number and specs.
or even 20log(max(abs(adc)/2048))
Hello,
I've added an external BPF which can be switched in and out
via USB and recorded the RX signals levels in the 20 m band with and
without it over a few hours; the idea was that the strong signals from the
broadcasting stations in the 41 m band could cause some second-order
intermodulation at 14 MHz. The antenna here is a good ol' 14AVQ trap
vertical in a noisy urban environment.
The filter passband goes from
about 12 MHz to 17 MHz and the 41 m band is attenuated by more than 30
dB; the losses in the passband are very small, about 0.3 dB or so. The
filter was switched by two mechanical relays to avoid adding any
intermodulation and also because using the PE4259 would have needed
doing another PCB, prototyping dead-bug style with SC-70 components is
no fun, hi.
A python script extracted the min, median and max values from every 48 kHz spectrum centered around 14.2 MHz computed by the Quisk routines, averaged these values over 30 seconds, saved them to a file then switched the BPF and started over again.
I was quite convinced that some of the noise floor was caused by the IM2 but this does not actually seems to be the case:
Hello Steve,
I tried to run the spectrum measurements for a longer
time but got some issues with one of the relays, that started to show
high losses in the "normally closed" position. Its behavior was quite
strange, just after switching it showed a loss greater than 10 dB then,
over a few seconds, it recovered to the usual practically zero losses.
Had to add some "wetting current", a few mA immediately restored the
normal functionality - maybe those surplus RF relays had switched
thousands of times already and needed come cleaning, hi.
I'm wondering if we should do this too with the relay on the H-Lv2b3? It's a completely different relay but likely not immune to this kind of problems.
Here is an updated noise floor graph, covering a whole day:
I can surely increase the gain significantly before clipping when using the bandpass filter, right now (11:00 pm local time) I can go until 18 dB without BPF and until 37 dB with the BPF
but will try to debug this on Monday or Tuesday.
Jim I hate to call on your time but it would be a great feature for HiQSDR please. I use this radio as my main rig and test bed but having said that I feel the Hermes-Lite will occupy an equal place as it matures.
Maybe for the Bandscope this could be automatically switched to 1.0 then switched back to whatever the user has selected before?
Hi group
FYI
I used Taka’s same 10x10cm format but with a different approach
- Absolutely NO “on board” logic, to accommodate any kind of switching command set : original J16 Hermes SPI protocol, I2C, bcd, decimal etc. Decoding will be located on another board to remain compatible with any kind of rig control
- Separate lpf and hpf board to fit any RF power in the TX / lpf path (50 W or 100W max ? not yet decided. So far, everything has been sized for a 20 W amp). Each filter, lpf and hpf, are located on different boards using same dimensions. Theses boards are mounted “piggy back way” to offer minimal crossband coupling
- T50 Toroids at least and 2 amps relays are used to limit losses on higher bands
In other words, a kind of Alexiares-like filter with a set of different control board to accommodate any kind of transceiver or any future evolution of the H.L.
I’ve not yet decided what kind of topology to use. So far, the M derived gives me the best impedance and phase and a nice frequency rejection on F2 and F3, but less overall rejection compared to a classical elliptic filter.
It’s a heavier and space consuming approach compared to an H.L. dedicated filter. But it’s the simplest way to stay compatible with the whole “hermes” family (H.L., Red Pitaya, Hermes/Angelia/Orion mk1..) or any other SDR, even an old "softrock/mobo" combination
the first "3D" brd is the lpf, the next is a simple hermes SPI decoder handling all the filter/antenna TX/RX and swr security switching.
I hope the final design will be tested before end of August.
73' Marc f6itu
Taka-san,
Well done! I was thinking of using some of my prior work as a
basis for a similar layout. Alas, I have been too busy lately to
do much more than follow the progress of this filter work. This
looks excellent!
I will be anxious to see how this performs operationally.
John W9JSW
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I suspect Jim has to compromise and is picking standard values available from manufacturers he likes. Did you fit your values to those commonly available and still see VSWR improvement
73, Taka ji1udd
SMD filters can have high losses, but do you expect 1 dB of loss across the whole passband?
Are you using a 2 layer or 4 layer PCB? Given the low price of 4 layer from Elecrow and the benefits of a nice uniform ground plane, I'd go with at least 4 layer for filter boards.
Thanks for all the filter board work! It looks like there will be several good options for the Hermes-Lite2. I want to make sure people can find and use this work.
Starting from the HL2 board, the first high pass filter is a 820pF series capacitor, a parallel 1.8 uH inductor, a series 470pF, a parallel 1.8uH, and a series 820pF. Then one of these filters consisting of a parallel C1, series L1, parallel C2, series L2 and parallel C3.
This sounds great! Thanks for all your work. For number 3, I was thinking the software would send a different set of relay selects to the I2C bus expander on TX and RX. Do you see a problem with that?
We may be able to move that to FPGA firmware so the software only has to send the RX and TX filter band select settings when the user changes them or changes bands.
The pin compatible 3V DPDT relays I have in my database are: EC2-3NU and NA-3W-K. They cost about a dollar a piece in quantity from aliexpress.com or ebay.com.
It bothers me that Jim has found it necessary to remove the HPF for TX to reduce filter losses.
This indicates that the HPF is in the wrong place and we should consider putting it only in the RX chain.
Jim, can you provide measurements for filter losses on all amateur radio bands 80M to 10M with and without the HPF?
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I've been thinking about filters today and wanted to bounce an idea off you and the group. I don't like the idea of having the HPF in the main TX/RX signal chain as it isn't useful for TX, requires a relay and is complicating the design with various filter interactions.
* Convert the current permanent LPF in the RX chain on the HL2 into two filters, selected by PE4259 devices. One selection will be a ~3MHz HPF, either the one you designed or the one Graeme designed and Taka tested. The other selection will be either pass through or the existing 35 MHz LPF, haven't decided yet. These filters can use smaller components than what you must use in TX. I have the room and FPGA line available to implement this.
**10/12M filter always in place, no relay. This would be the reconstruction filter for RX and would have to be good through VHF. The KX2 schematic has such a filter always inline, so I find it hard to believe it would contribute excessive loss.
This would require only 4-5 relays so there would be room on the board for higer quality low loss 10/12M filter.
* Filters derived from you work included on a single 15x10 cm board. (This has always been my end goal for a radio...)
Yes I got the information 100% and it will work well for me and just as a
cross check here is how I would set up 2 filters for a band.
assuming my 40M LPF is TX filter 3 and my 40M HPF is RX filter 2 the
settings would be as follows for 40M band button
TX = 0b10110010 RX = 0b00110010
On receipt of an I2C byte my decoding software would look at bit 7 to see
if in TX or RX mode and mask off the lower 4 bits for RX filter and the
upper 4 bits for TX filter dropping bit 4 in each case to control up to 8
RX and 8 TX filters which will more than match the Alex board which
switches 6 filters and 1 through pass for both transmit and receive. As you
are doing I will be storing TX filter, RX filter and ptt in a struct and
will only switch on a change. The I2C is on a high priority interrupt.
73, Graeme zl2apv
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