Traditional directional wattmeter uncertainty due to coupler Directivity

NanoVNA – how accurate does the LOAD need to be – part 1? discussed the importance of the calibration parts to accuracy of the NanoVNA.

Let’s digress for a moment at look at a directional wattmeter, a traditional way of measuring ReturnLoss and VSWR. This article examines the effect of coupler Directivity alone on uncertainty. There are other contributions to uncertainty, they are outside the scope of this article.

Directivity

Let’s review the meaning of directional coupler Directivity.

Above is a diagram from Mini-circuits. Continue reading Traditional directional wattmeter uncertainty due to coupler Directivity

Last update: 17th April, 2024, 5:38 PM

New e-delay feature in DisLord NanoVNA-H4 firmware v1.2.30

Recent articles discussed the use of e-delay to approximately compensate for cables connecting the DUT to the reference planes.

NanoVNA has had provision for an e-delay compensation for some time, it is a single value that is used to correct the s11 and s21 measurements.

It is very commonly the case that the optimal e-delay values for s11 and s21 compensation are different, so one needed to save a .s2p file for each of the two values and then merge the s11 measurements from its file with the s21 measurements from its file. The requires some work and risk of error.

I recently suggested to DisLord that provision be made in his NanoVNA firmware for specification of separate e-delay values for s11 and s21. He took the suggestion up and with days delivered a beta version with that facility.

This article documents an example use of the facility.

In this article, unless stated otherwise, reference to |s11| and ReturnLoss are to those quantities expressed in dB. Note that |s11|=‑40dB is less than |s11|=‑20dB. ReturnLoss and |s11| are related, ReturnLoss=‑|s11|.

Loss terms used are as defined at Measurement of various loss quantities with a VNA. Continue reading New e-delay feature in DisLord NanoVNA-H4 firmware v1.2.30

Last update: 18th April, 2024, 8:40 AM

NanoVNA – how accurate does the LOAD need to be – part 1?

A reader of EFHW transformer measurement – how accurate does the load need to be? asked whether the discussion applies more generally, in particular to the loads used for calibration and measurement with a VNA.

In this article, unless stated otherwise, reference to |s11| and ReturnLoss are to those quantities expressed in dB. Note that |s11|=-40dB is less than |s11|=-20dB. ReturnLoss and |s11| are related, ReturnLoss=‑|s11|.

Measurement 1

As a basis for discussion, let me offer an example measurement.

Above is a scan of a certain DUT after SOLT calibration of the NanoVNA. Continue reading NanoVNA – how accurate does the LOAD need to be – part 1?

Last update: 17th April, 2024, 6:19 AM

Using the NanoVNA to measure devices that have a UHF series connector – reader challenge

Using the NanoVNA to measure devices that have a UHF series connector left readers with a challenge:

An exercise for the reader: what would the e-delay need to be to compensate an s21 measurement if two identical cables were used to connect a UHF-UHF DUT?

Continue reading Using the NanoVNA to measure devices that have a UHF series connector – reader challenge

Last update: 14th April, 2024, 8:11 AM

EFHW transformer measurement – how accurate does the load need to be?

Several articles on this site use the following technique for measurement of transformer performance, and the question arises, how accurate does the load need to be?

Let’s set some limits on the range of ReturnLoss of interest. Measured ReturnLoss is limited by the instrument, and in the case of a VNA, its noise floor and the accuracy of the calibration parts used are the most common practical limits. That said, in practical DUT like an EFHW transformer, would would typically be interested in measuring ReturnLoss between say 10 and 32dB (equivalent to VSWR=1.05) with error less than say 3dB.

There are many contributions to error, and one of the largest is often the choice of transformer load resistor. This article explores that contribution alone.

2% load error

Let’s say the load resistor used is 2% high, 2450+2%=2499Ω. To measure ReturnLoss with such a resistor is to imply that the transformer is nominally  \(\frac{Z_{pri}}{Z_{sec}}=\frac{51}{2499}\) and ReturnLoss should be measured wrt reference impedance 51Ω.

To measure ReturnLoss wrt 50Ω gives rise to error.

Above is a chart of calculated ReturnLoss wrt Zref=51 (the actual ReturnLoss) and Zref=50 (the indicated ReturnLoss) for a range of load resistances, and the error in assuming RL50 when RL51 is the relevant measure. Continue reading EFHW transformer measurement – how accurate does the load need to be?

Last update: 14th April, 2024, 1:34 PM

Using the NanoVNA to measure devices that have a UHF series connector

From time to time I have a need to measure a device which has UHF series connectors.

UHF series connectors are not suitable for high accuracy measurements, and the problem is not simply that they are not ‘constant through impedance’ connectors, but the availability of reasonably priced calibration parts.

A simple solution when using short interconnecting cables at HF is to: Continue reading Using the NanoVNA to measure devices that have a UHF series connector

Last update: 13th April, 2024, 8:33 AM

Crystal substitute using si5351 – part 2

Crystal substitute using si5351 – part 1 described the first part of a series on an inexpensive crystal replacement using a si5351-A / MS5351M PLL chip and an ATTiny controller.

Above is an example pair of inexpensive modules, less than $10 for the pair (incl shipping). Both boards are powered from 5V, the left hand module is a ATTiny85 dev board, it has a small 3.3V regulator on board. The dev board uses a DIP chip, so it can easily be programmed in a device programmer and then inserted in the socket.

Above is a cold start of the module, error settles around +137ppm.

Development programming

 

Above is a screenshot of the EEPROM configuration data with two selectable configuration sets (burstsets) programmed. The ATTinyx5 has two pins for selection of one of four burstsets, the ATTinyx4 chips have four pins available for selection one of up to 16 burstsets, limited in both cases by the EEPROM available.

Bus Pirate v5

A Python script was written to parse the exported configuration from Clockbuilder Pro and create the Bus Pirate commands to be pasted into a terminal emulator (Teraterm v5). Bus Pirate 5 is not backwards compatible with v4 and v3, so the script accepts an argument to set the language version. The script also writes a binary file of the EEPROM image for use with a device programmer.

Teraterm was used as it allow specification of a pause at the end of each line to allow Bus Pirate to execute the command.

#!/usr/bin/env python
# coding: utf-8

from pathlib import Path
import csv
import os
import sys
import struct

print('\n'+Path(__file__).stem+' v1.02 20240330 Owen Duffy\n')

import getopt,sys

def usage():
print(Path(__file__).stem+" [-a hexaddress] [-m mversion][-o outprefix][-b bfilename][-p pfilename][-t toclen] ifile1 [ifilen]...")
sys.exit(2)

try:
opts,args=getopt.getopt(sys.argv[1:],"ha:b:m:o:p:t:v:",["help","address=","bfilenane=","mversion=","outprefix=","pfilename=","toclen=","verbose="])
except getopt.GetoptError as err:
# print help information and exit:
print(err) # will print something like "option -a not recognized"
usage()
sys.exit(2)

verbose=False

ifilesn=0
ifilenames=[]
if len(args)<1:
usage()
for arg in args:
ifilenames.append(arg)
print(arg,ifilesn)
ifilesn=ifilesn+1
chip=0xc0
toclen=ifilesn+3

p=Path(ifilenames[0])
pfilename=p.with_suffix('.pllldri')
bfilename=p.with_suffix('.bp')

for o, a in opts:
if o == "-v":
verbose = True
elif o in ("-h", "--help"):
usage()
sys.exit()
elif o in ("-a", "--address"):
chip=int(a,16)
elif o in ("-m", "--mversion"):
bpv=int(a)
elif o in ("-b", "--bfilename"):
bfilename=a
elif o in ("-o", "--outprefix"):
prefix=Path(a)
pfilename=prefix.with_suffix('.pllldri')
bfilename=prefix.with_suffix('.bp')
elif o in ("-p", "--pfilename"):
pfilename=a
elif o in ("-t", "--toclen"):
toclen=a
if toclen<ifilesn:
toclen=ifilesn
else:
assert False, "unhandled option"

bfile=open(bfilename,'w')
pfile=open(pfilename,'wb')
print('Writing: ',bfilename)
print('Writing: ',pfilename)

bus=0x1
lastaddr=-2

i=0
pbuf=[]
for ifilename in ifilenames:
#write pllldri eeprom image, use burst mode for speed / efficiency
toc=b''
burst=b''
pbuf.append(b'')
burstn=0
burstlen=0
burstsetlen=0

#write bus pirate commands
bfile.write(ifilenames[i])
if(bpv==5):
bfile.write(' BP5:\n\n')
bfile.write('m\n5\nn\n100k\nW\n3.3\n100\nP\ni')
else:
bfile.write(' BP4:\n\n')
bfile.write('m4 2 2 2\ne 2\nW\nP\ni')

with open(ifilename) as csvfile:
for line in csvfile:
rdr=csv.reader(csvfile)
for row in rdr:
if(len(row)>0):
if row[0].startswith("#"):
continue
addr=int(row[0])
data=int(row[1].rstrip('h'),16)
if(addr-1==lastaddr):
bfile.write(' 0x{:02x}'.format(data))
burst=burst+struct.pack('B',data)
burstlen=burstlen+1
else:
bfile.write('\n[ 0x{:02x} 0x{:02x} 0x{:02x}'.format(chip,addr,data))
if(burstlen>0):
burst=struct.pack('B',burstlen)+burst #prepend burstlen
pbuf[i]=pbuf[i]+burst #write to file buffer
burstsetlen=burstsetlen+burstlen+1
burstn=burstn+1
burst=struct.pack('BB',addr,data)
burstlen=2
#burstn=burstn+1
lastaddr=addr
# print(row)
# print('burstlen2: ',burstlen,burstn)
if(burstlen>0):
burst=struct.pack('B',burstlen)+burst #prepend burstlen
pbuf[i]=pbuf[i]+burst #write to file buffer
burstsetlen=burstsetlen+burstlen+1
burstn=burstn+1
pbuf[i]=struct.pack('BB',burstn,burstsetlen)+pbuf[i] #prepend burst list hdr
# print('burstlen2: ',burstlen,burstn)
bfile.write(' ]\np\nw\ni\n\n')

i=i+1

#write pfile header
buf=b'\x01\x03\x00'+struct.pack('B',chip)+struct.pack('B',len(pbuf))
#append toc
offs=5+toclen*2
i=0
for x in pbuf:
buf=buf+struct.pack('<H',offs) #append toc entry
offs=offs+len(x)
i=i+1
for j in range(i,toclen):
buf=buf+b'\xff\xff' #append empty toc records
#append burst sets
for x in pbuf:
buf=buf+x
pfile.write(buf)
pfile.close
bfile.close()

Above is the Python script.

HiZ> m

Mode selection
1. HiZ
2. 1-WIRE
3. UART
4. HDPLXUART
5. I2C
6. SPI
7. 2WIRE
8. LED
x. Exit
Mode > 5

Use previous settings?
I2C speed: 100kHz

y/n, x to exit (Y) > n

I2C speed
1kHz to 1000kHz
x. Exit
kHz (400kHz*) > 100k
Mode: I2C
I2C> W
Power supply
Volts (0.80V-5.00V)

Maximum current (0mA-500mA), <enter> for none
- SDA SCL - - - - - - GND
3.30V requested, closest value: 3.30V 0.0V 0.0V 0.0V 0.0V GND
100.0mA requested, closest value: 100.0mA

Power supply:Enabled
Vreg output: 3.3V, Vref/Vout pin: 3.3V, Current: 41.7mA

I2C> P
Pull-up resistors: Enabled (10K ohms @ 3.3V)
I2C> i

This device complies with part 15 of the FCC Rules. Operation is subject to the following two conditions: (1) this device may not cause harmful interference, and (2) this device must accept any interference received, including interference that may cause undesired operation.

Bus Pirate 5 REV10
Firmware main branch (2024-04-10T12:30:36Z)
RP2040 with 264KB RAM, 128Mbit FLASH
S/N: 2145260B134063E4
https://BusPirate.com/
Storage: 0.10GB (FAT16 File System)

Configuration file: Loaded
Available modes: HiZ 1-WIRE UART HDPLXUART I2C SPI 2WIRE LED
Active mode: HWI2C (speed)=(0)
Display format: Auto
Data format: 8 bits, MSB bitorder
Pull-up resistors: ON
Power supply: ON (3.3V/3.29V)
Current limit: OK (33.6mA/100.0mA)
Frequency generators: OFF

I2C> [ 0xc0 0x02 0x53 0x00 0x20
Vout: 3.29V/100.0mA max | Pull-ups: ON |
I2C START
038.3mA SDA SCL - - - - - - GND
3.3V [ 0xc0 0x07 0x00 3.2V 3.3V 3.2V 3.3V 3.3V 3.2V GND

I2C START
031.9mA0 ACK 0x07 ACK 0x00 ACK
3.3V [ 0xc0 0x0f 0x00 0x0c 0x8c 0x8c 0x8c 0x8c 0x8c 0x8c 0x8c 3.2V

I2C START
TX: 0xC0 ACK 0x0F ACK 0x00 ACK 0x0C ACK 0x8C ACK 0x8C ACK 0x8C ACK 0x8C ACK
007.0mAC ACK 0x8C ACK 0x8C ACK
3.3V [ 0xc0 0x1a 0xa4 0x3c 0x00 0x0f 0xff 0xb9 0x55 0xbc3.3V 3.3V

I2C START
TX: 0xC0 ACK 0x1A ACK 0xA4 ACK 0x3C ACK 0x00 ACK 0x0F ACK 0xFF ACK 0xB9 ACK
038.3mA5 ACK 0xBC ACK
3.3V [ 0xc0 0x2a 0x00 0x02 0x01 0x0f 0x40 0x00 0x00 0x003.3V 3.3V

I2C START
TX: 0xC0 ACK 0x2A ACK 0x00 ACK 0x02 ACK 0x01 ACK 0x0F ACK 0x40 ACK 0x00 ACK
035.6mA0 ACK 0x00 ACK
3.3V [ 0xc0 0x5a 0x00 0x003V 3.3V 3.3V 3.3V 3.3V 3.2V

I2C START
030.5mA0 ACK 0x5A ACK 0x00 ACK 0x00 ACK
3.3V [ 0xc0 0x95 0x00 0x00 0x00 0x00 0x00 0x00 0x00V 3.3V 3.3V

I2C START
TX: 0xC0 ACK 0x95 ACK 0x00 ACK 0x00 ACK 0x00 ACK 0x00 ACK 0x00 ACK 0x00 ACK
006.5mA0 ACK
3.3V [ 0xc0 0xa2 0x00 0x00 0x00 0x00 3.3V 3.3V 3.2V 3.3V

I2C START
024.4mA0 ACK 0xA2 ACK 0x00 ACK 0x00 ACK 0x00 ACK 0x00 ACK
3.3V [ 0xc0 0xb7 0x92 ] 3.3V 3.3V 3.3V 3.2V 3.3V 3.3V

I2C START
TX: 0xC0 ACK 0xB7 ACK 0x92 ACK
010.4mAP
3.3V p 3.3V 3.3V 3.3V 3.2V 3.3V 3.2V 3.3V 3.3V
Pull-up resistors: Disabled
I2C> w
Power supply: Disabled

I2C> i

This device complies with part 15 of the FCC Rules. Operation is subject to the following two conditions: (1) this device may not cause harmful interference, and (2) this device must accept any interference received, including interference that may cause undesired operation.

Bus Pirate 5 REV10
Firmware main branch (2024-04-10T12:30:36Z)
RP2040 with 264KB RAM, 128Mbit FLASH
S/N: 2145260B134063E4
https://BusPirate.com/
Storage: 0.10GB (FAT16 File System)

Configuration file: Loaded
Available modes: HiZ 1-WIRE UART HDPLXUART I2C SPI 2WIRE LED
Active mode: HWI2C (speed)=(0)
Display format: Auto
Data format: 8 bits, MSB bitorder
Pull-up resistors: OFF

Frequency generators: OFF

Above is a console log of Bus Pirate v5 programming of the PLL for development testing.

… more to follow.

Last update: 19th April, 2024, 1:16 PM

Crystal substitute using si5351 – part 1

Crystals have become very expensive (or so it seems), and the cost consigns some older radios to scrap. That said, I do recall buying crystals for $6 from Jan Crystals around 1967, which with Australian inflation over the period equates to $90 in 2023.

This article looks at an inexpensive substitute for the 1647kHz LSB crystal in a Codan 8525 transceiver which shipped as USB only in commercial service (yep, hams are out of step with the commercial comms world).

Above is a ‘si5351 module’ from Aliexpress for less than $3, about a tenth the price of the Adafruit module. Only one output will be used, the SMA jacks need not be used, but one was attached to channel 0 for testing the prototype. Continue reading Crystal substitute using si5351 – part 1

Last update: 12th April, 2024, 8:10 AM

1:49 EFHW transformer using a Jaycar LO1238 core – capacitor loss

An online expert talking about compensation capacitors and EFHW ferrite cored transformers opined:

If the evaluation is done solely by the effect on measured SWR, whether it is measured with a standard reflectometer or a VNA, then it is just as likely the capacitor is changing the losses in the transformer rather than actually adjusting the match.

“Just as likely” + gobbledygook, is this just hand waving on social media?

Let’s explore it using the calibrated model used in a series of articles starting with 1:49 EFHW transformer using a Jaycar LO1238 core – design workup.

Above is the SimNEC model as calibrated to bench measurement of a prototype transformer. The compensation capacitor Ccomp is specified as 100pF with Q=1000 (reasonable for a silver mica capacitor that is well suited to the application). Continue reading 1:49 EFHW transformer using a Jaycar LO1238 core – capacitor loss

Last update: 14th April, 2024, 1:34 PM