Roofing Filters

What "Roofing Filter" means to Elecraft

K3 Filters and Dynamic Range

A "Roofing filter" is simply a filter in the radio's first IF through which all signals must pass before they will be "seen" by later receiver stages. The narrower this filter is, the less exposure later stages will have. Thus a "narrow" roofing filter is desirable -- but "narrow" is relative, as I'll explain. 

The term "roofing filter" has most often been used in relation to triple- or quadruple-conversion receivers. Such receivers have an  IF above the highest RF band covered; it's typically something in the range of 30 to 70 MHz or higher. But "roofing" as a term should be interpreted as "protective," not "high in frequency." A roofing filter protects later stages, including amplifiers, mixers, narrower filters, and DSP subsystems, just as the roof on your house keeps rain out of all of the rooms. But a roofing filter can be equally at home at a low first IF, if that is how the radio is designed. It still provides the same protective function.

When we released the K2 in 1999, we never described our 1st IF crystal filters as roofing filters. We had only one IF, so the receiver model was simpler; there were no narrow filters at later stages that required protection.

But in 2007, we find that the term is in widespread use. Average hams now think of roofing filter bandwidths as the standard of comparison between receivers. This is why manufacturers have jumped through hoops to try to provide the narrowest possible roofing filters. Many operators have an understanding (justified) that a roofing filter that is wider than the communications bandwidth will not best protect the receiver's later stages. So the term now seems appropriate to use even in a radio such as the K2, K3, or Orion, all of which use low-frequency IFs (5 to 9 MHz).

In recent years, the roofing filter has become the centerpiece of receiver redesign:
Suppose that manufacturer "A" initially designed their receiver to use a 15- or 20-kHz roofing filter. Yes, this allows the receiver to handle NBFM and other wide modulation modes; it may also be selected to constrain the signal bandwidth ahead of a noise blanker or spectrum scope. But it comes at a price. If you're using CW mode, you'll have much narrower filters selected at the radio's 2nd and 3rd IFs. Yet the 1st IF roofing filter allows a broad swath of signals into the earlier stages. You don't need this energy in your passband. It can cause trouble.

Manufacturer "A," realizing they have a problem with dynamic range at close spacing, then announces that they've had a breakthrough: they can now offer a 6-kHz, or more recently 3-kHz roofing filter. This will certainly improve the situation for SSB and AM operation, but it still opens the barn door in CW or DATA modes, because the bandwidth is a factor of 10 wider than needed for communications.  

So why don't they offer much narrower roofing filters that can be switched in for CW and data modes, or at times when adjacent-channel SSB QRM is very high? It's because they can't make filters any narrower at such a high IF.

Enter the "down-conversion" rig (K2, K3, Orion, etc.). By converting to a low first IF, the designer can easily create narrow filters that are compatible with the required communications bandwidth. This is why we are offering filters with bandwidths as low as 200 Hz, as well as (in the future), variable-passband crystal filters.

And yes, these are still "roofing" filters, because they limit exposure (bandwidth), thus protecting later stages (in the K3 case, the IF amp, 2nd mixer, and DSP).

73,
Wayne
N6KR
CTO, Elecraft, Inc.

Return to FAQ

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've 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 passband. 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 spacing. As an example, we're seeing third order IMD DR numbers on 20M at 20 kHz spacing 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 bandpass filters, the core sizes used in those filters and inter-stage 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 spacing) 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 12 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