K3 Roofing Filters
From Zerobeat
Contents |
Roofing Filters Currently Available
KFL3A-200 200 Hz, 5-pole roofing filter 99.00 KFL3A-250 250 Hz, 8-pole roofing filter 125.00 KFL3A-400 400 Hz, 8-pole roofing filter 125.00 KFL3A-500 500 Hz, 5-pole roofing filter 99.00 KFL3A-1.0K 1 kHz, 8-pole roofing filter 125.00 KFL3A-1.8K 1.8 kHz, 8-pole roofing filter 125.00 KFL3A-2.1K 2.1 kHz, 8-pole roofing filter 125.00 KFL3A-2.8K 2.8 kHz, 8-pole roofing filter 125.00 KFL3A-6K 6 kHz, 8-pole roofing filter 125.00 KFL3B-FM FM b/w, 8-pole roofing filter 125.00
Not listed is the stock 2.7 kHz, 5-pole roofing filter that is included with the K3 and the KRX3 subreceiver. Elecraft has announced that buyers can elect to substitute the KFL3A-2.8K filter in place of the stock filter at the time of purchase and receive a $30 credit toward the purchase of the KFL3A-2.8K
Roofing Filter Response
Filter BW(-6dB) Shape Factor 200 224 4.0 250 370 2.1 400 435 2.1 500 565 3.1 1000 1063 1.6 1800 1913 1.5 2700 2910 2.9 2800 2888 1.6 6000 6125 1.5
Roofing Filter IMD
Here are filter IMD measurements from the Elecraft Lab posted to the reflector on Sept 13
Filter 20kHz 10kHz 5kHz 2kHz 200 Hz, 5 pole 100+ 100+ 100+ 95 250 Hz, 8 pole 100+ 100+ 100+ 95 400 Hz, 8 pole 100+ 100+ 100+ 95 500 Hz, 5 pole 100+ 100+ 100+ 94 1 kHz, 8 pole 100+ 100+ 100 94 2.7 kHz, 5 pole 100+ 98 92 n/a 2.8 kHz, 8 pole 100+ 100 93 n/a
These are typical numbers and may vary a couple of dB from radio to radio or filter to filter - Eric WA6HHQ
It should be mentioned that in a published review of the K3, G4AON observed degradation of close-spaced IMD measurements with the 400 Hz, 8 pole filter: "These figures are for a 400 Hz bandwidth with the 8 pole 400 Hz roofing filter, the rather surprising discovery was the dynamic range improved by almost 10 dB when the 2.8 KHz 8 pole filter was selected." G4AON's findings haven't yet been independently confirmed however.
WA6HHQ Roofing Filter Comments
There has been a lot of discussion and speculation on the list lately about the necessity for narrow filters ahead of the K3's DSP filtering and the impact of various filter bandwidths on receiver IMD. This in turn is creating a lot of confusion. I'd like to clarify this topic a bit.
Executive Summary: Our narrow filters improve both 3rd order IMD and Blocking Dynamic range. IMD does NOT degrade when using the narrower filters. Using the DSP as the -only- narrow filtering stage will degrade IMD and Blocking dynamic range. We designed the K3 system to use both the narrow crystal and DSP filtering stages in tandem for optimal dynamic range performance. (Basic filter recommendations below.)
Discussion: We have measured no serious degradation of IMD dynamic range when switching to narrower crystal filters on the K3. We carefully designed the K3 to avoid this. We isolate the crystal filters from preceding amplifier stages to optimize return loss outside the filter pass-band. Going from the 6 kHz to 2.8 kHz to 400 Hz to 200 Hz crystal filters does -not- significantly increase IMD in the K3. Just the opposite - Using narrow filters improves both third order IMD and Blocking IMD at close spacings. As an example, we're seeing third order IMD D.R. numbers on 20M at 20 kHz spacings in excess of 104 dB and 97 dB at 5 kHz with the 400 Hz filter ahead of the DSP. These numbers are much worse if you do not use a narrow crystal filter (400 Hz used for this test) ahead of the DSP.
We were very careful not to make the same mistakes made by other radio designs. Just as we did on the K2, which has a very similar first IF crystal filtering scheme, we have paid special attention to receiver gain stage balancing, proper isolation between the crystal filters and their surrounding amplifier stages and balancing the thresholds where hardware AGC and DSP AGC trade off their activation. Other areas of our design focus for optimal dynamic range are the PIN diode type and bias levels in the T/R switching area, the design of our front end band-pass filters, the core sizes used in those filters and interstage transformers and the IMD performance of the crystal filters. We chose INRAD as our OEM 8-pole crystal filter supplier because of their excellent filter performance.
In order to achieve a K3 blocking dynamic range (desense) in the 140 dB+ range, you -must- use a narrow crystal filter (400 Hz for closer interfering signal spacings) in front of the DSP. We use hardware AGC after the narrow crystal filter and ahead of the DSP to protect the DSP when signals inside the crystal filter exceed a 100 dB dynamic range. If you only use the 2.7 kHz stock filter for CW or data operation you will be significantly desensed once signals within that filter's bandwidth exceed about S9+25. This is before phase noise from the transmitting station becomes a factor. Not uncommon on 40M at night, during a contest or at a multi-op station. (Or every day in major cities ;-) Changing to a 400-500 Hz filter reduces blocking from signals 1-5 kHz away. I've personally confirmed this on the air with my K3 and the other commercial rigs we have here. when I've operated with the K3, or another DSP rig, on CW without using a narrow 400-500 Hz filter ahead of the DSP filtering, I frequently experienced desense (BDR) from nearby signals. Putting in the narrower crystal filter immediately cleaned it up. Using narrow crystal filters ahead of the DSP also reduces AGC pumping from static crashes on 80/160M etc.
My personal real-world operating -basic- filter recommendations? In a nutshell:
SSB: 2.7 kHz or 2.8 kHz
CW/DATA: 400Hz or 500Hz (Narrower for Data if you prefer)
AM: 6 kHz (And for wider SSB TX. We can select which filter you TX through and limit SSB b/w in the K3's DSP)
FM: FM b/w filter (I believe its in the 13 kHz range.)
Add narrower/wider filters as you prefer. I like to use the 1.0 kHz crystal filter when tuning a crowded band or listening to a pile up. I use the 2.1 kHz SSB filter on RX when someone crowds me on SSB. We provide 5 crystal filter slots per RX to accommodate a wide range of personal operating preferences.
Notes:
1. The stock 2.7 kHz filter is fine for most SSB operation. Since we also transmit through this filter, for wider TX bandwidth and slightly sharper RX stop-band skirts you may prefer the 2.8 kHz 8-pole filter. For wider 'hi-fi' SSB TX, you will need to transmit through the 6 kHz AM filter and let the DSP limit your ultimate bandwidth to something like 3-3.5 kHz.
2. For most CW operation I recommend the 400/500 and 200/250 Hz crystal filters. I personally find the 400-500 Hz easiest to listen to for most casual CW operation, but I use the 200/250 Hz filters to dig out the weak ones when there is a lot of nearby interfering activity.
Variable Bandwidth Roofing Filters
Here's the latest info Wayne N6KR posted:
One of the variable-passband filters we've been considering is one for SSB, with a range of about 1.4 to 2.2 kHz. It would have 5 poles rather than 8, but I'm sure it would work well in combination with the I.F. DSP. The MCU would automatically dial in the crystal filter bandwidth to match the DSP.
This would be a slot-saver for those want multiple SSB RX bandwidths. Once the CW variable-passband filter was also ready, you might have:
FL1 6.0 kHz FL2 2.7 or 2.8 kHz FL3 1.4-2.2 kHz (SSB variable passband crystal filter) FL4 0.4-0.8 kHz (CW variable passband crystal filter) FL5 200 Hz
Variable filters covering a wide range will have slightly greater ripple than fixed filters when they approach either end of their range, so we'd only use them in receive mode. In the example above, FL2 would be used for SSB/CW/DATA transmit and FL1 for AM transmit. Those who wanted FM and were less concerned about the narrowest possible CW/DATA bandwidths could shift all of the filters down one slot, bumping out the 200-Hz filter.
Use of Inrad/Yaesu 8.215Mhz Filters
Yaesu shares an identical i.f. frequency of 8.215Mhz with the K3. When asked whether Inrad/Yaesu filters could be used in the K3, Wayne N6KR responded:
The K3 uses completely different (and smaller) filter carrier boards. Your MK5 filters could in theory be removed from the existing carriers and put onto ours, but I'm not sure if we can supply empty carrier boards for customer use. Another consideration: We're holding all of our crystal filters to a high standard of dynamic range, and an older INRAD filter may not perform the same as the ones we supply. This is necessary because the K3 has higher performance than most of the rigs the filters were used in previously.
KBPF3
The KBPF3 is an array of pre-mixer bandpass filters rather than a post-mixer roofing filter, so it does not use any of the five available roofing filter slots. The KBPF3 is a separate PC board. A second KBPF3 can be installed for the subreceiver and likewise will leave the 5 subreceiver roofing filter slots available.
Using the FM Roofing filter for SSB receive
WB8YQJ March 23, 2008
External Links
Roofing Filter Article by N6KR and WA6HHQ
Roofing Filter Article by W2VJN
