Attenuator: 50ohm/81dB 1dB step

Created@2012/12/15

Attenuator Plan and select box

2 box, the one given following attenuation step: 

1dB  2dB  3dB  5dB

another :

10dB 20dB 20dB 20dB      

Design goal:

*RL>60dB at DC

*RL>40dB at VHF

*usable  to 70cm band

the die-casting aluminum box image:  ( 9cmx3.6cmx3cm)

Create drill Stencil

drill with Stencil

when finished, the drill Stencil will use as secondary PCB box provide excellent shield.

checking after drill finished

.

Get Ideal resistor from 5% series

use  %5  1/4W resistor, here is a method to get ideal resistor for attenuator: select a closed 5% resistor which big than the wanted value, then parallel another resistor to get perfect resistor. typically <1R to perfect attenuator value, even 0.5R. 

First of all, you should have at least a 4.5 digital DVM or equivalent device. 

for example, you want 5.769 ohm,

1) then pick 6.2R, in this step use a less value, ie, you 5% resistor had 6.0,6.1,6.2ohm, then use 6.0. 

2) then try to parallel a 68ohm resistor on it, check it with you DVM, if you get ie, 5.3ohm, if you accept this, done. if not

3) 5.3ohm<5.769, we should try use large resistor, try 75R,82R ....

here is all the matched pair for your start to get your ideal resistor, after the "=" is the value i get, and I'm very happy to get these perfect value resistor.

                                        perfect          closed

1dB:         shunt:           869.548         910||22k = 868    Input RL: >73dB

                 series:          5.769              6.2||68=5.75         Out  RL:>73dB

                                        perfect          closed

2dB:         shunt:           436.212        470||7.5k = 436.4    Input RL: >78dB

                 series:          11.615           12||430=11.60         Out  RL: >78dB

                                        perfect          closed

3dB:         shunt:           292.402        300||16k =292.5       Input RL: >75dB

                 series:          17.615           18||1.1k=17.6         Out  RL: >75dB

                                        perfect          closed

5dB:         shunt:           178.489        200||1.8k =178.6       Input RL: >60dB

                 series:          30.398           33||430= 30.49         Out  RL: >60dB

-----------------------------------------------------------------------------------------------

                                        perfect          closed

10dB:         shunt:          96.248        100||3.9k = 96.3     Input RL: >70dB

                   series:         71.151          75||1.3k= 71.1        Out  RL: >70dB

                                        perfect          closed

20dB:         shunt:          61.111         62||3.3k =61.10      Input RL: >70dB

                   series:         247.502       270(265.3)||3.9k(3.83)=247.8       Out  RL: >70dB

Making secondary PCB shield box fit into the die-casting box

drill mask used as another PCB box's panel, then we need more PCB parts to form a box. the following image show how to make many same size parts one time.

When finished all parts, put them into die cast box, checking if the dimension is suitable. 

NOTE:

1. use wire to connect bottom and the top layer of the PCB

2.  use PCB strip to connect the switch for better performance at high frequncy.  you should make through hole for inter connect.

3. use copper tape seal the bottom 

DC checking

Use a 4 1/2 digital DVM, a 50R load, a battery to check the DC resistance and the DC attenuation. this ensure all of them connected properly and given a low Frequency attenuation reference.

battery ->  attenuator   -> 50R load

Attenuation=20*log(Vout/Vin)

1. resistance check

terminated by 50R, test another port resistance

1db/2db/3db/10db/20db   :  50.05R

5dB:  50.10R

2. DC attenuate

1dB:    in: 1.2983 out:1.1572      0.999 dB

2dB    in:1.2978   out: 1.0316     1.994 dB

3dB    in: 1.2976  out: 0.9186     3.000 dB

5dB    in:1.2975   out: 0.7289     5.009 dB

10dB  in:1.2980   out: 0.4101    10.007dB

20dB  in:1.2977   out:0.12931    20.03 dB

@2003/9/22

few day ago i finished the 8307 Power Meter, which work really good up to 470Mhz(which i can test by a handy transceiver with 40dB pad attenuator) , so can verify the high frequency performance.

10Mhz testing(pending)

dam good.

144Mhz testing(pending)

really good.

455Mhz testing(pending)

1-10dB is fine, but with 20dB, only get 18~19db attenuation.    refer following sections.

compensation the step attenuator up to 500Mhz

@2003/9/22

Here is the simulation with LTspice,  a typical axial 1/4 Watt resistor might have 8nH parasitic inductance and 0.3pF pakage ((which should less than 8nH, i cut  the lead is damn short).  with these i do get the seems fit the reality performance.

how to compensate this? learn something from the AD8307 compensate method, i can use a 3pF capacitor with long lead bend to a 5mm diameter inductor, witch should around 8nH~9nH.  I got this simulate result:

Simulation tell me such things should work, let's check  it tonight!   so much  effort to construct this device, but axial version seems hard to work to 500Mhz.

compensation Version

Final Result:

below the line is compassionate version, accuracy improved 2.x dB from 140M to 470Mhz.

(column 0 means the signal strength when enumerator set to 0, -10 represent the signal when 10 db switch in)

*1- 10dB attenuation and are useful up to 500Mhz,  less than 0.5dB error (up to 500Mhz).

*20dB attenuation are accuracy up to 150Mhz, error less than 0.5dB after compensation.

*from 200 to 450Mhz  error less than -1dB.

*470Mhz  get +1 error attenuation.

*  1-10dB + 2X20dB  is useable, but 3x20dB won't work...... (might because signal leak?)

*Compensation is work but limited, seem, it's hard to tune it to cut at 900Mhz and keep error < 1dB