Custom 700 Hz K3 Filter

Custom 700 Hz K3 Filter

Custom 700 Hz K3 Filter

Custom 700 Hz K3 Filter

700 Hz filter photo.
Credit N1JR

700 Hz is per­fect for scan­ning CW bands and for 500 Hz dig­i­tal for­mats.  And even though some users are quite hap­py hav­ing it as their only CW fil­ter, we be­lieve the big­gest sell­ing point for 700 Hz is that it yields an ideal two-fil­ter “tool­kit”, when paired with the INRAD 400.

“… ab­so­lute­ly won­der­ful …” (K2UM)

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700 Hz filter passband roll-off plot
Brochure

Description

The bro­chure in­cludes some unique in­for­ma­tion …

  • A brief his­to­ry of how the 700 Hz fil­ter came to be.
  • A ge­ner­ic 8-pole Che­by­shev crys­tal fil­ter sche­mat­ic, which we had hoped to re­place with an of­fi­cial INRAD-sup­plied draw­ing.  (Un­for­tu­nate­ly their sche­mat­ic is pro­pri­e­tary.)
  • Our un­of­fi­cial shape fac­tor cal­cu­la­tion.

… as well as some in­for­ma­tion that can al­so be found else­where on our site …

PDFs – Published in Three Formats (Revised 5 AUG 2013)

News

8 July 2014 – The fil­ters have ar­rived (ahead of sched­ule) and will be­gin ship­ping to­mor­row.  Our in­ter­nal des­ig­na­tion for this man­u­fac­tur­ing lot is #2714, iden­ti­fy­ing the week and year it was re­ceived.

2 June 2014 – We com­mis­sioned the next batch sans a pre­or­der wait­ing list, to short­en the pro­cure­ment cy­cle.

26 May 2014 – The lat­est man­u­fac­tur­ing lot has sold out.

9 May 2014 – The fil­ters have ar­rived from INRAD.  Our in­ter­nal des­ig­na­tion for this man­u­fac­tur­ing lot is #1814.

29 January 2014 – The next pro­duc­tion run has been com­mis­sioned.  We ex­pect it will ar­rive on Fri­day May 16th, give or take a week.

22 January 2014 – We are cur­rent­ly ac­cept­ing pre­or­ders towards the next pro­duc­tion run.

18 October 2013 – Batch­es four and five have ar­rived and been shipped.

6 August 2013 – The fourth batch has sold out and we have com­mis­sioned a fifth batch.

10 July 2013 – Batch #4 has been com­mis­sioned.

19 June 2013 – Batch three has sold out.

17 February 2012 – Wayne con­firmed that Elecraft is not much clos­er to of­fer­ing switch­a­ble dual-band­width fil­ters, than they were when the K3 was first an­nounced.  (“Still a gleam in my eye” as he put it.) 

What Others Have Said

Direct email feedback …

Ethan Miller,
K8GU
I’m ex­treme­ly de­light­ed … it’s a great all-around CW fil­ter for DSP band­widths be­tween 500 and 700 Hz.”
Dave Hammond,
N1LQ
“WOW!  What a find! … a keep­er for sure.”
Just more pleas­ant … cruis­ing the CW bands …”
“This is the ideal gen­er­al-pur­pose CW fil­ter.”
Mike Lee,
AA6ML
“the best $[..] that I’ve spent in a long time”
“… be­ing able to "sam­ple" the band … with­out get­ting tired … is, in a word, PRICELESS.”
Scott Monks,
AAØAA
“… very help­ful for … all but the big­gest pile-ups.”
“… well worth the pur­chase price and the ser­vice was out­stand­ing.”
“… I heart­i­ly rec­om­mend this fil­ter.”
Rick Tavan,
N6XI
“… 700 is good for … split pile­ups.  I’m com­fort­a­ble with my 400s for eve­ry­thing else.”
Greg Buhyoff,
K2UM
I love my 700 Hz fil­ter!  It is ab­so­lute­ly won­der­ful for "lis­ten­ing wide" … wid­er than that is too wide and 500 Hz is too nar­row.  Damn good fil­ter width for me.”
Stefan Reyers,
DJ7AO
“I Love CW with my K3 with your fil­ter … it has been the right choice :-)”
Steve Smith,
MM0SAJ
“… ex­cel­lent …”
“Now that I see how good it is, I have to have [one for] the sub re­ceiv­er …”
Don Wilhelm,
W3FPR
I like it, and would NOT be will­ing to part with it.  It is my pref­er­ence for tun­ing the CW bands and al­so for da­ta modes.”
Rodrigo Ferreira,
CT1BXT
“Very hap­py with it.  I can com­pare it against my 400 and it seems [to have] bet­ter au­dio.”
“… that could [just] be … my per­cep­tion [it’s] noth­ing sci­en­tif­ic.”
Steve Campbell,
GM4OSS
“De­light­ed with my fil­ter …”
Keith Hamilton,
N8CEP
“Very good fil­ter!  One must re­mem­ber:  it is a ROOFING fil­ter, not the final band­width fil­ter.”
Pierre Desmarteau,
VE2KY
“… a love­ly band­width … like it very much.”
Julius Fazekas,
N2WN
I like the 700 Hz, … it helps cut the crud when … S&P­ing … def­i­nite­ly a keep­er.”
Jack Nelson,
K5FSE
“My K3 [al­so] has 400 and 250 [Hz] fil­ters … you said it best when you lik­ened them to view­ing the world through a [drink­ing] straw.  … when I’m just [cruis­ing] the bands … 700 [Hz] … is sure more fun.”
John Shadle,
NE4U
I like the 700 Hz fil­ter a lot.  I use it for scan­ning the bands and then use the 400 Hz fil­ter for QSOs.”
“… the miss­ing link be­tween the 1500 and 350 Hz fil­ters … [now] I can de­crease the band­width on da­ta mode con­tin­u­ous­ly from 2800 to 50 Hz …”
Ray Spreadbury,
G3XLG
“… I am very hap­py … it fits nice­ly be­tween my 1.8 kHz & the 250 Hz fit­ted in my K3.”
Tony Scandurra,
K4QE
“Thanks … for mak­ing these great fil­ters avail­a­ble!”
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Links to ad­di­tion­al com­ments in the Elecraft® Re­flec­tor ar­chives …

Terms of Sale
  • The 700 Hz fil­ters are not re­turn­a­ble.
  • If you can­cel your or­der be­fore it ships, you will in­cur a $29 (per fil­ter) or­der can­cel­la­tion fee.
  • In the un­like­ly event that your or­der is lost or dam­aged by the post­al ser­vice, we re­serve the op­tion to re­im­burse your pur­chase in lieu of re­plac­ing the lost or dam­aged prod­uct(s).

Frequently Asked Questions

What’s so special about 700 Hz?

On Its Own Merits

  1. If you are just start­ing out, 700 Hz is an ex­cel­lent “first” CW fil­ter for deal­ing with mod­er­ate­ly crowd­ed band con­di­tions.  And un­less you’re a se­ri­ous CW or RTTY con­test­er, 700 Hz is prob­a­bly the only CW/da­ta fil­ter you’ll ev­er (re­al­ly) need!
  2. If you are just start­ing out, 700 Hz is an ex­cel­lent “first” CW fil­ter for deal­ing with mod­er­ate­ly crowd­ed band con­di­tions.  And un­less you’re a se­ri­ous CW or RTTY con­test­er, 700 Hz is prob­a­bly the only CW/da­ta fil­ter you’ll ev­er (re­al­ly) need!
  3. 700 Hz is a good fit for 500 Hz dig­i­tal-mode for­mats.
  4. Band con­di­tions per­mit­ting, many CW operators con­sid­er a pass­band width in the neigh­bor­hood of 650 to 750 Hz to be ideal for scan­ning.

Considering the Big Picture

700 Hz “pairs nice­ly” with INRAD’s 400 Hz fil­ter and al­so with their 1,500 Hz fil­ter.  By this we mean that there’s op­ti­mal spac­ing be­tween them.  (Not too much, and – just as im­por­tant­ly – not to lit­tle.)  This “pair­ing nice­ly” as­pect of 700 Hz was one of the ma­jor fac­tors that led to our com­mis­sion­ing INRAD to de­sign and man­u­fac­ture this fil­ter for us.

Ex­pand­ing on the no­tion of “pair­ing nice­ly”, a use­ful analo­gy is to think of your fil­ters as “tools”.  In this con­text, choos­ing which fil­ters to get is like out­fit­ting a tool­kit with an as­sort­ment of screw­driv­ers in prep­a­ra­tion for a DX­pe­di­tion, but with the stip­u­la­tion that you may only bring five with you.  Un­less you only ex­pect to use these screw­driv­ers to re­pair eye­glass­es, you’d be wise to se­lect a wide as­sort­ment of si­zes.

The same rule ap­plies to K3 fil­ters.  You might be able to jus­ti­fy choos­ing the four nar­row­est (500, 400, 250, and 200 Hz) for your K3, if it will only ev­er be used for run­ning un­der the most ex­treme CW con­test con­di­tions, with nu­mer­ous close-in S9+25 sig­nals.

The draw­back of this strat­e­gy is that there is a di­min­ish­ing re­turn when­ev­er any two fil­ters have fair­ly sim­i­lar widths, ul­ti­mate­ly re­duc­ing the “con­tri­bu­tion” each fil­ter can make to your K3’s max­i­mum “po­ten­tial” crys­tal fil­ter­ing flex­i­bil­i­ty.  (The ex­treme case – to dem­on­strate this point – would be to imag­ine in­stall­ing two iden­ti­cal fil­ters in your main or in your sub-RX!)

Yes, un­der some band con­di­tions you might be able to dis­cern a dif­fer­ence be­tween two 8-pole fil­ters of very sim­i­lar widths.  Spe­cif­i­cal­ly, be­tween INRAD’s 500 and 400, or be­tween their 400 and 250.  But the dif­fer­ence is un­like­ly to be so dra­mat­ic as to war­rant in­stall­ing both of them.

Please don’t mis­un­der­stand … these three fil­ters – the INRAD 500, 400, and 250 Hz – are per­fect­ly fine choi­ces; it’s just that in­stall­ing all three, isn’t three times bet­ter than only hav­ing one.

Compared to INRAD’s 1000 Hz Filter

At first glance the 1000 Hz might seem to be more flex­i­ble, since it can al­so be used for 1000 Hz dig­i­tal for­mats.  That is, un­til you take a clos­er look and re­al­ize this fil­ter is only about 975 Hz wide at mi­nus 3 dB.  Thus a nar­row SSB fil­ter might be bet­ter suit­ed for these modes; es­pe­cial­ly for weak sig­nal work.  But more im­por­tant­ly, a 700 Hz fil­ter is sure to out­per­form a 1000 Hz fil­ter, at pass­band widths be­low 700 Hz but above the point where your next nar­row­est fil­ter is au­to­mat­i­cal­ly se­lect­ed.

Compared to INRAD’s 500 Hz Filter

500 Hz might ac­tu­al­ly be the bet­ter sin­gle-fil­ter “so­lu­tion” for ca­su­al CW con­test­ing.  Es­pe­cial­ly if your pri­mary ob­jec­tive is to only use one fil­ter slot for CW.  Co­inci­den­tal­ly, 500 Hz is both too wide and too nar­row to “pair well” with most oth­er CW fil­ters any­way.  Fur­ther, 500 Hz is some­what of a one-size-fits-all com­pro­mise and as such, it doesn’t do any­thing par­tic­u­lar­ly well.  Spe­cif­i­cal­ly:

  1. It’s wid­er than op­ti­mal for con­test “run­ning”.
  2. It’s wid­er than op­ti­mal for con­test “run­ning”.
  3. Be­ing only about 480 Hz wide at mi­nus 3 dB, it’s a rath­er tight fit for 500 Hz dig­i­tal for­mats.
  4. It’s a bit nar­row for ra­pid scan­ning.
  5. It’s a bit nar­row for ra­pid scan­ning.

Still, a 500 Hz fil­ter might work just fine for you, pro­vid­ed that:

  1. You al­so have a pan­a­dapt­er, and nev­er scan “by ear” any­way.
  2. You al­so have a 1,000 Hz fil­ter (for scan­ning and for 500 Hz dig­i­tal for­mats).
  3. The only dig­i­tal modes you ev­er op­er­ate are RTTY and PSK.
Why aren’t Elecraft and/or INRAD offering this width?

Our guess is that there mul­ti­ple con­trib­ut­ing fac­tors, in­clud­ing:  be­cause they have high­er pri­or­i­ties; be­cause the up-front ef­fort/cost to add it to their prod­uct lines is pro­hib­i­tive; and be­cause the de­mand isn’t suf­fi­cient to make do­ing so worth their while.

What is the filter’s actual (minus 6 dB) bandwidth?

785 ± 15 Hz.  We es­ti­mate the shape fac­tor at 1.7.  Our orig­i­nal in­ten­tion was to of­fer a 750 Hz fil­ter, some­thing mid­way be­tween INRAD’s 500 and 1000 Hz.  We spec­i­fied 700 Hz, know­ing the mi­nus 6 dB width would be sev­er­al per­cent wid­er.  In hind­sight, per­haps we should have spec­i­fied 675 Hz?  How­ev­er, at the time our pri­mary con­cern was that we didn’t want some­thing too sim­i­lar to INRAD’s ex­ist­ing 500 Hz fil­ter, and we have ful­filled on that in­ten­tion.

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When will they be back in stock?

They are right now … but only be­cause the lat­est batch ar­rived much ear­lier than ex­pect­ed.  (This batch is much small­er than usu­al and will like­ly sell out be­fore the end of Au­gust.)

Can I buy them directly from INRAD?

These cus­tom fil­ters are avail­a­ble ex­clu­sive­ly through unpcbs.com.  INRAD does not of­fer them for sale on their web­site.  If they were avail­a­ble di­rect­ly from INRAD, there would be no rea­son for us to of­fer them.

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App Note #8 – 7th Batch: Plus 13 Hz Offset
plus 10 Hz offset graphic

Over­all, the fil­ters in the Ju­ly 2014 batch have a cen­ter fre­quen­cy of 8215.013 kHz, i.e. a plus 13 Hz off­set.  En­ter [ +0.01 ] in­to the “FLx FRQ” Config me­nu, to com­pen­sate for this +0.00016% de­vi­a­tion.

App Note #7 – 6th Batch: Plus 10 Hz Offset

Over­all, the fil­ters de­liv­ered in May 2014 have a cen­ter fre­quen­cy of 8215.027 kHz, i.e. a plus 10 Hz off­set.  En­ter [ 0.01 ] in­to the “FLx FRQ” Config me­nu, to com­pen­sate for this +0.00012% de­vi­a­tion.

App Note #6 – 4th & 5th Batches: Plus 27 Hz Offset

Over­all, the fil­ters de­liv­ered in Oc­to­ber 2013 have a cen­ter fre­quen­cy of 8215.027 kHz, i.e. a plus 27 Hz off­set.  En­ter [ 0.03 ] in­to the “FLx FRQ” Config me­nu, to com­pen­sate for this +0.00033% de­vi­a­tion.

App Note #5 – 3rd Batch: Minus 45 Hz Offset

Over­all, the fil­ters in the Feb­ru­ary 2013 batch have a cen­ter fre­quen­cy of 8214.955 kHz, i.e. a mi­nus 45 Hz off­set.  En­ter [ -0.04 ] in­to the “FLx FRQ” Config me­nu, to com­pen­sate for this -0.00055% de­vi­a­tion.

App Note #4 – 2nd Batch: Minus 20 Hz Offset

Over­all, the fil­ters in the June 2012 batch have a cen­ter fre­quen­cy of 8214.98 kHz, i.e. a mi­nus 20 Hz off­set.  En­ter [ -0.02 ] in­to the “FLx FRQ” Config me­nu, to com­pen­sate for this -0.00025% de­vi­a­tion.

App Note #3 – Insertion Loss Compensation

Up­on add­ing a new fil­ter, you should use the “FLx GN” Config me­nu to com­pen­sate for the rel­a­tive dif­fer­enc­es in in­ser­tion loss across your fil­ters.  Where­as the K3 man­u­al rec­om­mends 1 or 2 dB for 400 and 500 Hz fil­ters, we es­ti­mate that the dif­fer­ence be­tween the 2.8 kHz and 700 Hz fil­ters is ac­tu­al­ly clos­er to 3 dB.  (Spe­cif­i­cal­ly, mi­nus 3 dB at 2.8 kHz ver­sus mi­nus 6 dB at 700 Hz.)  Still, 2 dB is prob­a­bly a good start­ing point.

Note that the ob­jec­tive here isn’t to com­pen­sate for all of the in­ser­tion loss.  Rath­er, it is to use the min­i­mum gain nec­es­sary so sig­nal lev­els do not fluc­tu­ate ob­jec­tion­a­bly when switch­ing fil­ters.  Most like­ly, you’ll find the best set­ting for 700 Hz is be­tween 1 and 3 dB.  Get­ting the best re­sults might en­tail chang­ing the loss com­pen­sa­tion of one or more of your ex­ist­ing fil­ters.

App Note #2 – 1st Batch: Missing Hardware

In the rush to send the fil­ters, all do­mes­tic or­ders were shipped with­out the re­quired mount­ing hard­ware, i.e. (1) 4-40 zinc pan-head Phil­lips ma­chine screw and (1) lock wash­er.  Most of you will have al­ready the re­quired hard­ware on hand, or will be able to ob­tain it lo­cal­ly.  How­ev­er, in­ter­na­tion­al cus­tom­ers may ex­pe­ri­ence dif­fi­cul­ty find­ing non-met­ric parts.  We will pro­vide this miss­ing hard­ware up­on re­quest.

Caution!  Be sure to use a 3/16" (4.8 mm) or 1/4" (6.4 mm) long screw!  Any­thing long­er might ex­tend in­to the fil­ter and dam­age it!

App Note #1 – 1st Batch: Plus 60 Hz Offset

Over­all, the fil­ters in the De­cem­ber 2010 batch have a cen­ter fre­quen­cy of 8215.06 kHz, i.e. a plus 60 Hz off­set.  En­ter [ 0.06 ] in­to the “FLx FRQ” Config me­nu, to com­pen­sate for this +0.0007% de­vi­a­tion.

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See Also
  • Filter Plots – K3 First IF – INRAD 8-Pole and Elecraft® 5-Pole – Crys­tal Fil­ter Re­sponse Curves.
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