[Elecraft] K3 Crystal Filters and Dynamic Range - D.R. impact and my basic recommendations

[Eric Swartz - WA6HHQ, Elecraft]

Whew! The last week has been a tidal wave of interest, orders and questions.

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.


Finally, until we start shipping K3s, Wayne, myself and the rest of our 
design team will be consumed with getting the K3 into production and 
making sure we meet all of our performance goals for the rig.  I'm sure 
we will be optimizing the design right up until we ship ;-) We want the 
K3 to be as good as it can get when we deliver it to you.

We will not be able to respond to every posting or engage in technical 
debate on the reflector.  We'll occasionally post as time permits and we 
plan to post additional detailed test numbers as we have a wider range 
of production K3s to test against. We want to make sure the numbers we 
post are accurate, conservative and correlate well with the test methods 
used by the ARRL, Sherwood etc. (Just as we did on the K2.)

73, Eric  WA6HHQ
Elecraft