How does Postman generate oAuth 1.0a signatures?

I'm attempting to connect to an oAuth 1.0 authenticated endpoint but the signature that Postman creates does not match the signature we expect in our code. As far as I can see all the parameters that I have added are correct (Url, consumer_key, consumer_secret, nonce, timestamp, HMAC-SHA1) but Postman just seems to generate a different signature with these details. Does anyone know how Postman creates the signature?